Greetings!
We appreciate your post in the Apple Support Community. Changing the paper format in Pages templates, especially for books, can be a bit challenging. You can try adjusting the paper size through the Page Layout Inspector by opening the document, enabling the Layout view, and modifying the paper size in the Document section. However, keep in mind that some templates may have fixed layouts that aren't easily altered. If the adjustments don't work seamlessly, creating a new document with the desired paper size and manually transferring content might be a more effective solution. Remember to save your changes and be prepared to make manual adjustments to elements for the best results.
For English speakers, we have a dedicated community right here: discussions.apple.com. Since your message is in English, we warmly invite you to join us in that community.
Thank you!