You can access your master pages from the Home Tab or by clicking "See All" on the My Books page. From there, you can click on any master page to edit it, sync changes to all books, or delete it.
If you would like this update to appear in all books you have used it in, then click the Apply Changes button at the bottom right corner of the page after making your changes.
You can learn more about setting up and using Master Pages in this tutorial: https://www.atticus.io/how-to-create-a-template/