God does not ever change His mind regarding the unchanging goodness of His character, or faithfulness of His promises and His Word, and this is what the Bible refers to when it is said that He does not change, or does not change His mind. He is not unreliable and untrustable like man.
God changes His mind only under one Biblical circumstance. This is repenting from GOOD into BETTER (because God does not do evil like man) for His mercies' sake... this is when He wants to give a just judgment, which is good, but changes into better that grace might abound and this is indescribable goodness, the forgiveness of God... a core example of this is Old Covenant being changed into New Covenant. The change is here not because of God but because of us. We need to understand what we have really deserved (which is just administration of God's wrath) to understand the greatness of God's grace and how good God really is. Some examples:
Psalms 106:45 And he remembered for them his covenant, and repented according to the multitude of his mercies.
Jonah 3:10 And God saw their works, that they turned from their evil way; and God repented of the evil [evil here doesnt mean sin but the intended destruction of Ninevah], that he had said that he would do unto them; and he did it not.
Judges 2:18 And when the LORD raised them up judges, then the LORD was with the judge, and delivered them out of the hand of their enemies all the days of the judge: for it repented the LORD because of their groanings by reason of them that oppressed them and vexed them. ---God changes His mind for His mercies' sake...
2 Corinthians 3:11 For if that which is done away [the Old Covenant] was glorious, much more that which remaineth [the New Covenant of grace] is glorious. ---God changing things from good into much better...