How to delete backup photos or videos

Backup where? iCloud or iTunes?

iTunes Backup - Photos and Videos cannot be deleted exclusively. You need to delete the entire backup.

iCloud Backup - iCloud backup doesn’t include information that is synced and stored in iCloud, like Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, Messages*, shared photos, iCloud Photos*, Health data, call history, and files you store in iCloud Drive.

*-If you had turned on Messages in iCloud & iCloud Photos, your content is automatically stored in iCloud. 

If you delete Photos & Videos from the iPhone it is gone from iCloud too and vice-versa.