How do I change a book's genre, sub-genre or audience after it's created/imported?

On the book's page, click the genre badge under the title (it shows a small pencil on hover). A "Book metadata" editor opens with genre, sub-genre and target audience.

Changing them is safe at any time — they guide cover design and AI tools, and never modify your text. (Added in v1.5.1; before that, genre was fixed after creation.)