‘Jon 3:10 When God saw what they did, how they turned from their evil way, God relented of the disaster that he had said he would do to them, and he did not do it.
The context of Jonah 3 is that the prophet, Jonah, is sent to Nineveh against his will. He preaches unmitigated destruction against the city. The people repent in the hopes that God will revoke the disaster that He had declared against them.
In Jonah 3:10, God sees that the people repent. As a result, God repents of what He said He would do. The text emphasizes this by saying “and He did not do it”. The author of Jonah is making it clear that God responds to people’s actions. God was going to destroy Nineveh. God said He would destroy Nineveh. But God does not ultimately do what He said He “would do”.
This text is not portraying a God who does not change, knows the future exhaustively, or even who believes His own decrees must come to past. Instead, as Jonah declares against God (in a critical manner), God is a God who repents.’
source: Christopher Fisher (godisopen).