Each time you save a draft, or publish or unpublish a page, the system saves a version of the page's content. To see what pages have changed recently, open the "Reports" submenu near the bottom of the admin menu, then choose "Site History".
On the Site History page, you can search for pages by title, action (e.g. publish), user, and date. The title on each line links to the current edit page for that page. From there you can make more edits, preview the current draft, and go to the page's full history by clicking on the View History link in the Status sidebar (opened via the circular
i icon in the upper right).
To see a list of each time the current page has changed, click the "View History" link at the top of the Status sidebar in the page editor (opened via the circular
i icon). Or, if you are on the Sitemap, there is a link to each page's history under the "More" dropdown menu.
The history for each page includes a list of all drafts, as well as entries for each time the page was published or unpublished. You can compare each version with the version before and after it. If you would like to revert the current page to a previous version, click the "Review this version" link on the draft you would like to recover.
"Review this version" will take you to an editor with the chosen version of the page loaded. Since this is not the current draft of the page, you will see an orange band at the top with the version information (date and who created it) and the actions bar at the bottom of the page says "Replace current draft" rather than the usual "Save draft".
At this point you can edit and preview as usual. When finished, you can replace the current draft or publish your recovered version.