One of the most mind-boggling things about the Bible is that God, in spite of His omniscience, is nevertheless reactionary. He knows exactly what is going to happen, and yet He seems to change His mind a lot, even though it is explicitly stated that He does not (Numbers 23:19). How does this work? Is this a contradiction, or are we missing something here?
Although the Bible does talk of God relenting, it is actually just simplifying the issue. The philosophical term is that God "condescends" to us. In other words, He speaks to us at our level. You might think of this as how a teacher educates a child. I remember a while back, my friend sent me a video of the same physicist explaining gravity to people of varying levels of education. She did not use the same language with the child (the child looked about 7-9) as with her fellow physicist. In the same way, the Bible does not talk to us as if we were fellow members of the Godhead. In fact, the Bible tells us the Spirit intercedes for us when we pray using groanings no man can utter (Romans 8:26).
So, God talks to us at our level. What we perceive as God changing His mind is actually God proceeding as He always knew, and in some cases foretold He would. This is because although God is omniscient, we are certainly not. From our perspective, the future has not yet happened. From God's perspective, it has.
This has many asking how free will can therefore exist? If our future choices have already been made, how can we have the freedom to make them? The "simple" answer to this is that although they have been made from God's perspective, they were free choices when they were made from ours.
Consider the case of Moses. God always knew Moses would intercede for Israel, yet Moses was the one who chose to do so. If Moses had chosen not to, God would likewise have known that Moses would not. This would possibly alter how God would have acted. After all, while God is not human, humans are nevertheless made in His image. We are similar to Him in many ways. We make plans, just as He does. We "course correct", just as He does. The difference, of course, is nothing can thwart Him. There are no hiccups, no nasty surprises, not so much as a tiny unaccounted variable for God. He knows exactly what will happen at any given moment, along with every possible alternative. He knew every word that would come from Moses' mouth, and every word that could come from Moses' mouth. And that's just putting it lightly. God's knowledge is so far beyond our comprehension, it is literally impossible to describe it with words. But to give you an idea, consider that He has full control over every atom in our universe at every given moment. We, by contrast, can't even count the grains in a jar of sand.
And so we see there is no contradiction. It's just that, when talking to man, God speaks like a man. Why would we expect any different? He wants to get His message across, not leave us confused and terrified. Thus, we should not be surprised when God condescends to us. Rather, we should be grateful.