Jer 26:13 Now therefore mend your ways and your deeds, and obey the voice of the LORD your God, and the LORD will relent [repent] of the disaster that he has pronounced against you.
The setting is the reign of Jehoiakim, and the people appear to not be walking “in the law” of God (v4). Jeremiah prophesies against them, and they then wish to kill him. A similar prophet of God, Uriah, is killed in this passage. It is not clear if the people listened to Jeremiah, as the King sought to kill him.
Jeremiah 26:13 serves as a typical call and response. God calls the people to repent. If the people repent, God will then change His plans for those people. God will respond as humans respond.
Jeremiah 26:13 fits the formula Jeremiah sets out in Jeremiah 18:
Jer 18:8 and if that nation, concerning which I have spoken, turns from its evil, I will relent [repent] of the disaster that I intended [thought] to do to it.
Although this conditional does not seem activated by the people, Jeremiah treats it as if it is a reality. He calls on the people to change. He says God will change. Earlier, God commissions Jeremiah, wondering if perhaps the people might listen (v3). God has no set future plan and is willing to change based on changing circumstances.
Another good example is Nineveh which was not destroyed in fourthy days (read the book of Jonah).