How do I rename, move, leave out or delete a section I added to my book?

From version 1.9.24, hover the section's row on the book's page. The rename icon changes the heading it prints with. The trash icon deletes it after you confirm: its text and its versions are kept in the book's folder, under "deleted", and it is removed from the translated editions too. To move it, use Reorder above the chapter list, or the "Include in book and order" list in the Export card, where you can also untick it to leave it out of the book without deleting it.