If it's a .capx, then use a program such as winzip or 7zip to unpack the .capx to a new folder. Edit the .caproj file using a program like notepad and find the xml line:
<saved-with-version>16900</saved-with-version>[/code:ioy6vv72]
Change the version from 16900 to 16800 and save.
Double click the .caproj to launch into C2 and re-save as .capx.