When PmWiki is called with '?action=diff', it displays a summary of past edits on a page. Each past edit is shown in a box which shows lines added, changed or deleted during that edit in a before & after format.
Below each box is a "Restore" link. Clicking the link will open an edit box with the page as it was before that edit. You can make changes or simply click Save to restore the text.
There are two additional options specific to PageHistory:
A page's history is kept for the number of days given by the $DiffKeepDays variable (set by the site's wiki administrator). When a page is edited, any page history information older than this value is automatically discarded.