To make a backup of a template, you can copy it. This article will show you how to copy a template in the Template Details menus.
To back up the sites you create, we highly recommend Publishing after any significant changes you make to the site. Whenever you Publish your site, you are creating a Published Version that you can, then, revert back to if you decide to. However, please be cautious when publishing a site without authentication as search engines might display it publicly in its search result.
You can return to a previously Published Version of your site by selecting Revert in the editing left editing panel.
This article provides more information on how to revert between Published Versions of your site, along with specific examples of how this feature functions.