Finding and deleting only the backup photos may be difficult because there is no special album for backed up photos.
Searching Google Photos for #autobackup may help to find all the photos in the backup, but we're not sure that method works; please give it a try. (These directions do not apply to the "Backup & Sync" app because we are not sure what you would need to do at Step 6.)
If you want to clear everything (photos and videos) from your Google Photos account and start a new backup using Google Photos Backup, please follow the directions below. Clearing all photos and videos will also clear out any albums you may have. If you had shared links to those albums, the links will no longer work.
Make sure your original photos are safely stored somewhere else (like an external hard drive).
Download everything using Google Takeout as described here.
Delete all your pictures and videos; directions are here.
Deleted pictures no longer appear at Google Photos Library and Albums, all synced devices, and at Google Drive
Empty the Trash
Uninstall the Google Photos Backup app
Delete the following folder on your computer
On Windows: %localappdata%\Google\Google Photos Backup\ (copy/paste the path into the address bar in "Windows Explorer" or "File Explorer")
On Mac: ~/Library/Application Support/Google/Google Photos Backup/
Download and reinstall the Google Photos Backup app. The GPB installation file can be picked up at TC Peter's OneDrive here:
Set up the backup app to backup your computer files and devices