Where is the backup of my appendices saved when I regenerate them?

From version 1.9.4, before a full Regenerate appendices rewrites anything, every current section is copied to a folder named "appendices-backup" inside the book's folder, in a sub-folder named with the date and time, one plain Markdown (.md) file per section. You can open these files in any text editor or in Word. The tickbox "Copy the current sections to a backup folder first" in the confirmation window is on by default; untick it if you do not want the copy. The previous text of every rewritten section is also kept in that section's Versions inside the app. To find the book's folder, use the Reveal option in the book's workspace.