Jer 42:10 If you will remain in this land, then I will build you up and not pull you down; I will plant you, and not pluck you up; for I relent of the disaster that I did to you.
The context of Jeremiah 42:10 is that Johanan and a pro-Egyptian, anti-Babylonian contingent are headed to Egypt after defeating the forces of Ishmael (who has assassinated Gedaliah, a Babylonian appointee). Johanan is worried that Babylon will indiscriminately kill his people in retaliation although they supported Gedaliah. En route to Egypt, Johanan encounters Jeremiah. Jeremiah tells them that God commands them to stay in Israel and not go to Egypt. Jeremiah couples this with both curses and blessings, blessings if they stay and curses if they leave.
Within this speech, Jeremiah states that God is sorry for the disaster that He brought upon them. John M. Bracke writes:
1. God is “sorry” for the disaster brought upon Judah (v. 10). The same Hebrew word here translated “sorry” is used elsewhere in the book to indicate God’s changed mind (or heart: 18:8, 10; 26:3). The sense here is not that God has made a mistake in destroying Judah but regrets what has happened (even though Judah gave God no other options) and is eager for something different. God has plucked up and torn down, but, that accomplished, the Lord is ready to build and plant.
2. God will “save,” “rescue,” and have “mercy” on the remnant of Judah (vv. 1112; compare 30:8, 9 11; 31:7, 20). These assurances are all linked to Babylon. Judah is no longer to fear Babylon (v. 11) because God has a new function for Babylon in relation to Judah. God has used Babylon to express anger and judgment through the exile of 587 B.C., so there has been reason to fear Babylon (or at least how God would use Babylon). Following 587 B.C., Babylon will have a different role as the agent of God’s saving, rescue, and mercy. Verse 12 summarizes the point: ”I [God] will grant you mercy, and he [Babylon] will have mercy on you, and restore you to your native soil.”
Naturally, Johanan ignores Jeremiah and calls him a liar. The contingent flees to Egypt (bringing Jeremiah with them), ignoring God’s promises to build them in the land of Israel. God’s anger is aroused again (v44:8).
In light of this, God’s repentance in Jeremiah 42:10 was failed attempt at reconciliation. God being sorry for what He had done (or alternatively God’s renewed commitment to Israel’s prosperity) was never actualized. The recipients continued on in disbelief and soon began serving other gods, which in turn changed God from open to reconciliation to being consumed with anger (v44:11-14)