Most people are at least somewhat familiar with the story of Jonah. Jonah was told by God to go to Nineveh and tell the inhabitants that their city would be destroyed in 40 days. There were no "ifs", "ands", or "buts". However, from the king on down to the lowest member of society, the inhabitants repented, fasted, and prayed and said among themselves, "If we repent, possibly God may not destroy us and our city." (Jonah 3: 5-9)
"Then God saw their works, that they turned from their evil way; and God relented from the disaster that He had said He would bring upon them, and He did not do it." (Jonah 3:10)
God had planned to destroy Nineveh, but the people's repentance brought about a change in God's plans. There is nothing in the entire book of Jonah that even indicates that God was going to do anything other than destroy them and their city. It is clear from the text and the narrative of the story that God did indeed change His mind. This is a fairly common occurrence in God's Word. Theologians may disagree but the Biblical narrative remains true!