Malachi 3:6; and Genesis 6:6,7; Exodus 32:14; Jonah 3:10

God does not change
(Malachi 3:6) - "For I, the Lord, do not change; therefore you, O sons of Jacob, are not consumed."

God does change
(Genesis 6:6,7) - "And the Lord was sorry that He had made man on the earth, and He was grieved in His heart. 7And the Lord said, "I will blot out man whom I have created from the face of the land, from man to animals to creeping things and to birds of the sky; for I am sorry that I have made them."
(Exodus 32:14) - "So the Lord changed His mind about the harm which He said He would do to His people."
(Jonah 3:10) - "When God saw their deeds, that they turned from their wicked way, then God relented concerning the calamity which He had declared He would bring upon them. And He did not do it."

When God says that He does not change, He is speaking about His nature and character. But this does not mean that He cannot change how He works with people throughout history.

When we see God changing His mind, we are seeing it from a human perspective. Since God knows all things from all eternity, He has always known the ultimate plan that He would carry out--even the plan to "change His mind." As we have seen in Jonah's account of Nineveh, they repented and God relented from the destruction that was to come upon the inhabitants. Of course, God knew this would happen and instituted the warning to them in order to bring about their repentance. There is no mystery here.

Source:
http://carm.org/bible-difficulties/genesis-deuteronomy/does-lord-change-or-not