In addition, a lack of memory space is a factor to consider. If you believe you've accumulated a large number of files that have already
consumed your device's internal storage, you should
clean up by deleting all superfluous files.
- To begin, go to Settings-> General-> Storage & iCloud Usage-> Manage Storage and check your iPhone's memory. Go to Settings-> General-> iPhone Storage if your iPhone is already running iOS 11.
You can relocate some of your crucial data to other storage media, such as iCloud or iTunes backup, if necessary. After you've freed up some space on your iPhone's internal storage, reboot it again, and then try to upgrade iOS.