You can use "page history" at the top of the page to compare two previous versions of a page and/or to revert to a previous version of a page.

Or, you can click on one of the versions to see the page as it was in that version:

Notice that Mariel has clicked "save" three times while editing. This has created three versions of the page. In fact PBworks (like MSWord and most online tools) saves automatically as you write. You only need to press "save" if you leave the page.

However, when you click and view a preceding version you are offered the option at the top:

Beware - this is one of the rare things by which you can permanently loose something on pbworks. If you revert to an earlier version, then all the preceding versions are lost, so PBworks kindly gives you a warning: