1. Left click the gear icon to the top left of the edit banner
2. Select Versions from the menu
3. This will load the list of version for the page you are editing. The one highlighted in lighter blue is what is live for the public.
4. Everything else in the list is a previous edit of the page. 
5. To compare version, tick the boxes of each version you want to compare and the right hand side will tab each version to show you the two versions

If you make a number of changes and publish a page but then need to remove those changes and put it back to a previous version simply hover over the previous version of the page and click the small arrow.

This will display a menu with the following options:

1. Approve - makes that version of the page live to the public
2. Duplicate - Duplicates that version of the amends
3. New page - Allows you to create a new page based on this version of the page
4. Delete - Deletes the information stored within this version

In order to keep your website database efficient we do not store version from all of time since the site launches but in general you will have the latest 10-20 versions of a page available to roll back.