This goes back to a post earlier where I explained that God is different from us. I agree for humans, the future is unknown. This is not true of God. You should read Isaiah 46:9-10.
4 And Jonah began to enter into the city a day's journey, and he cried, and said, Yet forty days, and Nineveh shall be overthrown. This is the message from God. Nineveh will be overthrown in forty days.
5 So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them. The people believed God. They believed the God of Israel was going to destroy them in forty days.
6 For word came unto the king of Nineveh, and he arose from his throne, and he laid his robe from him, and covered him with sackcloth, and sat in ashes.
7 And he caused it to be proclaimed and published through Nineveh by the decree of the king and his nobles, saying, Let neither man nor beast, herd nor flock, taste any thing: let them not feed, nor drink water:
8 But let man and beast be covered with sackcloth, and cry mightily unto God: yea, let them turn every one from his evil way, and from the violence that is in their hands. The people repented from their evil and cried out to God. It was their only hope as verse 9 states.
9 Who can tell if God will turn and repent, and turn away from his fierce anger, that we perish not?
10 And God saw their works, that they turned from their evil way; and God repented of the evil, that he had said that he would do unto them; and he did it not. God saw. God repented. God did not do what he said he was going to do.
There is only two choices:
1. God lied about destroying them.
2. God changed his course of action based upon man's repentance and crying out to him when faced with his word.
It really is that simple.