If you are trying to update your iPhone and are unable to, it could be because you haven’t erased and restored your iPhone to its original settings. Use the following steps to properly update your iPhone:
-
Back up everything on your iPhone included all saved data, settings, contacts, calendars, notes and camera roll. Since iTunes won’t back up everything, make sure you back up to iClouds.
-
Reset your iPhone and make sure all data is erased.
-
Update to latest iOS.
-
Restore iPhone through iTunes.
-
Restart.
-
Sync data, settings, contacts, calendars, notes and camera roll back onto phone via and/or iCloud.
Resource: Apple Update