I replaced a word everywhere by mistake. Can I undo it?

Yes. Before "Replace" (on the book's page, next to Codex) changes anything, BookyAI keeps the old text. For a chapter, open it in the editor, press Versions, and put back the version from just before the replacement. Every file that changed (chapters, outline, Codex notes, the Codex itself and the book's details) is also copied, as it was, into a folder named "replace-" with the date and time, inside the "history" folder of the book. Open the book's folder from the book's page (Files) to find it. For a single word, the quickest undo is often another Replace, the other way round.