To make this change, you go to Settings > Writing Mode. Here is an article about the Writing Mode page:

http://help.leanpub.com/en/articles/4748124-where-is-the-writing-mode-page-for-my-book

Here, you can choose which branch you use to preview, and which branch you use to publish. These can be the same branch or different branches, and they can be named any legal Git branch name.