Unable to upgrade iPhone 7 from iOS 12.0 to 15
I have an old iPhone 7 on iOS 12.0 and I want to use it as a backup. However it won't let me upgrade to the latest iOS 15. It just constantly says it can't verify the update.
I have tried going into storage, deleting the update and downloading it again but I get the same error
"Unable to Install Update" "An Error Occurred Installing (null)".
Any suggestions on how I get around this. Is it because you can't go from 12 to 15 in one go?
iPhone 7