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GetSimple creates a backup of every page every time you make a change. Currently, it only keeps one revision at a time. The way you can tell if a page has a backup is to check at the bottom of the Page Edit screen.
If a page backup is available, then you can either click that link, or head over to Backups → Page Backups and find the page title that you are looking for. Once you clikc into that page's backup, you are presented with a few actions you can perform:
If you click on “RESTORE”, then this page becomes the current page, and the current file becomes the backup. “DELETE” obviously deletes this backup, and leaves the current page intact.