How do I edit media files in an iBooks Author project?

Question

macOS

Hello, I have a task to translate an iBooks book into English. Most of the sources are available, but, alas, not all. The book has several multimedia keynote presentations as widgets, and if plain text is easy to replace, then these presentations are a real problem: some of the presentations have no source, so I don’t know how to extract the original from iBooks Author for editing (English translation ) and further introduction back.