my iphone 5 won't back up.
when i plug my iphone 5 into the charger, it should back up, but it's not, because, rather than overwriting previous backups, it is saving each backup as a separate version, and has eaten all of my storage space. I want to set it up so that each successive backup overwrites the previous. can i do that? and how can i do that. i'm kind of tech stupid, so the directions have to be understandable for a 5 year old. thanks!
iPhone 5, iOS 9.3.3