Unfortunately, standard users can not restore a previous version of a page. You can, however, view a previous version and copy and paste the contents.
Click on the name of a page in SiteExecutive.
Near the top-left you will see a dropdown menu labeled "Version".
Click on that and select a previous version to view it.
- "Archive" - previously published versions of the page.
- "Active" - the page in it's currently published/live state.
- "Editable" - any edits to the most recently published version that have been saved but not yet published.