Is it possible to change the status of a language from Vernacular to a Source Language?
You may need to be more explicit about what you’re trying to do…
But in case I have guessed correctly…
If you simply want to move the text you currently have in the book content in the Edit tab into the source bubbles so you can enter a new language into the content of the book, you can go to the Collections tab, Settings, and change Language 1 to the new language. When you go back to Edit, the content will be blank and the existing text will be in the source bubbles.