Showing under Manage Storage where? The Mac? The iOS device? Are you running Sierra or higher on the Mac? If so, go to >About This Mac>Manage>iOS Files. There you will see all of your iTunes Backups listed. You can remove them from there, but be aware, that once you delete them, they CANNOT BE RETRIEVED.
If you are referring to iCloud Backups, then, again whether you have Backups turned on or not, you can delete previous backup by following the path I provided in the first post. And as with the iTunes Backups, you cannot retrieve them one you have deleted them, so make sure that is what you want to do.
Best,
GB