To sync changes made to a master page to all books:
Click "Apply Changes" in the bottom right corner of the content editor.
A dialog box will appear prompting you to choose whether to apply the changes to all books or just the current one.
Select "Apply to all books" to sync the changes across all books that use the master page template.
Click "Apply current changes" to confirm and implement the synchronization.
You can learn more about setting up and using Master Pages in this tutorial: https://www.atticus.io/how-to-create-a-template/