The reasons for the cause of this failed update are many, but there is a solution to help you solve them.
If a problem occurs during the upgrade process, the first thing you will have to do is to wait for a couple of hours and try to update again. The failure might have been cause by Apple Update Servers and during this time it should be successful.
You can also restart your iPhone/iPad and reset your access point to the internet. Follow this simple steps:
- Goto Settings
- Reset Network Settings
If you don't want to reset Network Setting and lose your saved WI-Fi password and configurations, you can install iOS 10 directly using an IPSW firmware file. Download the version for your iPhone or IPad here and update with iTunes.
In some cases Update is downloaded, but not installed. You can just Force Reboot Device to complete update: Hold down Home + Power button for a few seconds until you see the Apple logo on the screen.