If you look in the console at the list of backups for an Upsun site, you'll note that they only exist for the main environment and there is no way to browse them. This makes restoring a backup something of a crapshoot. Fortunately there's a better way. 

1. From the CLI, checkout the main branch.
2. upsun backup:list to get a list of available backups.
3. Get the ID of the one you want and upsun backup:restore --target=stage backup-ID.

This will restore the backup to the stage environment. If the stage site is correct, you can simply merge to main. Alternately, you can specify the name of another environment as the argument to the --target option and copy whatever files you need from there.