God is immutable! However, in several passages of the Bible, there is an account of how God changed His mind.

Amos 7:1-6 “I had a vision from the Sovereign LORD. In it I saw him create a swarm of locusts just after the king’s share of the hay had been cut and the grass was starting to grow again. In my vision I saw the locusts eat up every green thing in the land, and then I said, “Sovereign LORD, forgive your people! How can they survive? They are so small and weak!” The LORD changed his mind and said, “What you saw will not take place.” I had another vision from the Sovereign LORD. In it I saw him preparing to punish his people with fire. The fire burned up the great ocean under the earth and started to burn up the land. Then I said, “Stop, O Sovereign LORD! How can your people survive? They are so small and weak!” The LORD changed his mind again and said, “This will not take place either.” (“Bible Gateway Passage: Amos 7:1-6 – Good News Translation” 2019)

Amos prayed and “The Lord changed His mind about this. ‘It shall not be,’ said the Lord.”

Some versions said God “relented” which means became less severe or less harsh.

Exodus 32:6-14 “Early the next morning they brought some animals to burn as sacrifices and others to eat as fellowship offerings. The people sat down to a feast, which turned into an orgy of drinking and sex. The LORD said to Moses, “Hurry and go back down, because your people, whom you led out of Egypt, have sinned and rejected me. They have already left the way that I commanded them to follow; they have made a bull-calf out of melted gold and have worshiped it and offered sacrifices to it. They are saying that this is their god, who led them out of Egypt. I know how stubborn these people are. Now, do not try to stop me. I am angry with them, and I am going to destroy them. Then I will make you and your descendants into a great nation.”
But Moses pleaded with the LORD his God and said, “LORD, why should you be so angry with your people, whom you rescued from Egypt with great might and power? Why should the Egyptians be able to say that you led your people out of Egypt, planning to kill them in the mountains and destroy them completely? Stop being angry; change your mind and do not bring this disaster on your people. Remember your servants Abraham, Isaac, and Jacob. Remember the solemn promise you made to them to give them as many descendants as there are stars in the sky and to give their descendants all that land you promised would be their possession forever. So, the LORD changed his mind and did not bring on his people the disaster he had threatened.” (“Bible Gateway Passage: Exodus 32:6-14 – Good News Translation” 2019)

Moses pleaded, which is a form of praying. Exodus 32:14 “So the Lord changed His mind about the harm which He said He would do to His people.” (“Bible Gateway Passage: Exodus 32:14 – New American Standard Bible” 2019)

God does not change. When the Bible says that God relented or changed His mind, it is not saying that something new has happened and now He is thinking in a different way. God knows all things. Instead, it is describing God’s attitude changing. Not changing because events have caught Him off guard, but because now this aspect of His character is more fitting to be expressed than it was earlier. Everything is laid out according to how He has ordained. His nature does not change. From eternity past, God has known exactly what was going to happen. He has infinite and complete knowledge of everything that will ever happen.

When Abraham pleaded for God to save Sodom and Gomorra for the sake of the righteous people, God saved Lot and his daughters. Read Genesis 28:20 ahead. Gen 19:29 “When God destroyed the cities of the plain, God remembered Abraham and sent Lot out of the middle of the overthrow, when he overthrew the cities in which Lot lived.” (“Bible Gateway Passage: Exodus 32:14 – New American Standard Bible” 2019)

We must persevere in prayer through opposition. Daniel prayed but the angel that God sent him was delayed by a demon.

In Daniel 10:11-13, the angel is speaking: “And he said to me, “O Daniel, man greatly beloved, understand the words that I speak to you, and stand upright, for I have now been sent to you.” While he was speaking this word to me, I stood trembling.
Then he said to me, “Do not fear, Daniel, for from the first day that you set your heart to understand, and to humble yourself before your God, your words were heard; and I have come because of your words. But the prince of the kingdom of Persia withstood me twenty-one days; and behold, Michael, one of the chief princes, came to help me, for I had been left alone there with the kings of Persia.” (“Bible Gateway Passage: Daniel 10:10-13 – New King James Version” 2019)

God is never surprised. We never take God by surprise. He already knew that less than 10 people were righteous in Sodom. But He allowed Abraham to ask Him to change His mind if 50 or 40 righteous were found and persevered in asking, lowering the number each time.

Prayer changes things. God uses prayer as a means to bring His work to pass. God uses our prayers as a means of carrying out His will.

God not only invites us to pray but even commands us to. The effective prayer of righteous people accomplishes much. But these things do not change God’s mind because God has never had to change His mind from the foundation of the world (Ephesians 1:11 “All things are done according to God’s plan and decision; and God chose us to be his own people in union with Christ because of his own purpose, based on what he had decided from the very beginning.”) Ephesians 1:11 – Good News Translation” 2019) He knows all. Prayer is the form of worship and communication God selects for us to grow in our relationship with Him.

God acts in accordance with our choices. God tells us of the cautionary nature of some of His declarations and the fact that He will act in accordance with our choices.

Jeremiah 18:7-10 “If I announce that a certain nation or kingdom is to be uprooted, torn down, and destroyed, but then that nation renounces its evil ways, I will not destroy it as I had planned. And if I announce that I will plant and build up a certain nation or kingdom, but then that nation turns to evil and refuses to obey me, I will not bless it as I said I would.”  (“Bible Gateway Passage: Jeremiah 18:7-10 – New Living Translation” 2019)

Also, through Jonah, God had told Nineveh He would destroy the city in forty days (Jonah 3:4). However, Nineveh repented of their sin (verses 5–9). In response to the Assyrians’ repentance, God relented: “He had compassion and did not bring upon them the destruction He had threatened” (verse 10). (“Bible Gateway Passage: Jonah 3:4-10 – New Living Translation” 2019)

The fact that God changes His treatment of us in response to our choices has nothing to do with His character. In fact, because God does not change, He must treat the righteous differently from the unrighteous. If someone repents, God consistently forgives; if someone refuses to repent, God consistently judges.

God commands us to pray. Jesus said: Matthew 7:7-11 “Ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you. For everyone who asks receives, and the one who seeks finds, and to the one who knocks it will be opened. Or which one of you, if his son asks him for bread, will give him a stone? Or if he asks for a fish, will he give him a serpent? If you then, who are evil, know how to give good gifts to your children, how much more will your Father who is in heaven give good things to those who ask him!” (“Bible Gateway Passage: Matthew 7:7-11 – New King James Version” 2019)

Mark 11:23-26 “Truly, I say to you, whoever says to this mountain, ‘Be taken up and thrown into the sea,’ and does not doubt in his heart, but believes that what he says will come to pass, it will be done for him. Therefore, I tell you, whatever you ask in prayer, believe that you have received it, and it will be yours. And whenever you stand praying, forgive, if you have anything against anyone, so that your Father also who is in heaven may forgive you your trespasses.” (“Bible Gateway Passage: Mark 11:23-26 – New American Standard Bible” 2019)

1 John 5:14 “This is the confidence which we have before Him, that, if we ask anything according to His will, He hears us.” (1 John 5:14 – New American Standard Bible” 2015)

God wants commitment

“Then Jacob made a vow, saying, “If God will be with me, and keep me in this way that I am going, and give me bread to eat and clothing to put on,  so that I come back to my father’s house in peace, then the Lord shall be my God.  And this stone which I have set as a pillar shall be God’s house, and of all that You give me I will surely give a tenth to You.”” (“Bible Gateway Passage: Genesis 28:20-22 – New King James Version” 2019)