Check i OS software version: You might get an error message that reads, "This device isn't eligible for the requested build", while trying to update or, downgrade to a firmware which Apple has stopped signing.
Some users may have to use i and reset their Apple ID password manually, which is frustrating but it can sometimes fix the issue as well.
If you can’t update wirelessly on your i OS device, you can update manually using i Tunes on a computer that you trust.
If your computer is using Personal Hotspot on the i OS device that you’re updating, connect your computer to a different Wi-Fi or Ethernet network before you update.
While the download may take longer for you due to extreme traffic, this is a different issue.
When users are trying to update the new mobile operating system while restoring from the backup, they are unable to make the upgrade.