You can view the version history of a page by clicking on the timestamp below the page title:
Alternatively, you can click on the dropdown menu and select "View version history":
Reverting to a Previous Revision
To revert to a previous revision, first select the desired revision in the left sidebar. Next, click the "Revert to this version" button near the top of the page:
Content highlighted in red means it was removed in this version. Text highlighted in green means it was added.