How do I change the parts the AI proposed before my book is written?

Turn on "Review the outline first" in New Book. From version 1.9.24, when the outline is ready, each part appears in it as a row of its own. Click the part's title to rename it, write "A few lines for the part's page (optional)" in the box under it, or use the trash icon to delete it (its chapters stay and join the part before it). The layers icon on a chapter, "Add part here", starts a new part at that chapter. Use a chapter's up and down arrows to move it past a part row, into or out of a part. A part with no chapter under it is left out when you save. Then approve the outline and the book is written with your parts.