The changes that you make in your editing interface are automatically saved. This means that the latest version of a site will open when you click on the Edit button from your Sites and Templates dashboard.
Whenever you Publish your site, you also create a Published Version. Therefore, if you decide you want to return to a previous Published Version, you can do so by selecting Revert on the top corner of your Site Editor.
Be sure to preview the version before selecting Revert, to ensure that it is, indeed, the version you want.
You can switch between the published versions as much as you'd like (for example, you can go from your most recent Published Version back to the very first Published Version, and then back to the most recent again) but keep in mind that any local changes - changes made without publishing to save them - will not remain once you revert to another Published Version.