How do I delete apple watch ad from screensaver?

Hard drive level Library/Screen Savers. Also check your user/home Library.

Do a backup.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library/Screen Savers.