Malachi 3:6 “I the LORD do not change. So you, O descendants of Jacob, are not destroyed.â€
James 1:17 "Every good and perfect gift is from above, coming down from the Father of the heavenly lights, who does not change like shifting shadows.â€
Numbers 23:19 “God is not a man, that He should lie, nor a son of man, that He should change His mind. Does He speak and then not act? Does He promise and not fulfill?â€
The line you gave, along with a few others in Genesis (6:6) and Exodus (32:14) are improper translations of the Hebrew word. The word is properly translated to regret (though, repent does fit the bill, but it can be misleading). We can regret and be sorry for things we have done, it doesn't mean we reverse them or would change our minds about them.
I regret ever introducing myself this girl named Erin but I'd never, ever, change that.
In that verse from Jonah, God is talking about how He felt bad for what He was gong to do to the Ninevites, but because they repented for their sins He showed mercy, and as you may recall, mercy is totally within God's frame of mind! Furthermore, if we look at that entire chapter in context, God was threatening the Ninevites, He is omnipotent, He knows when a threat will work.
What you are quoting is a line that many atheists use to try and show God is contradicting in His Word and scripture. This is wrong, it is a human projection that people are trying to fit onto God.