To complete this action you must first log in to your ArcSite web/user site here: https://user.arcsite.com/

Once there, click into the Project Folder for the drawings in question, choose the drawing, and then you'll see the different versions available on the right. You can download the Version of your choosing and re-import it into ArcSite to work on that instead of the current version you are on.

This article showcases details on completing this action. https://support.arcsite.com/en/articles/7258282-importing-existing-plans-pdfs-into-arcsite