Multiple users have reported issues with failing updates - this being a widespread issue which is (a) reported to be known to Apple - and (b) is now understood to have been resolved.
There are a number of potential remedies that are being reported to be successful in correcting the issue, including:
- if available, switch to a different WiFi network
- if available, switch to your phone hotspot
- restart your home broadband router - then force-restart your device
- connect your device to a PC/Mac - and attempt update using iTunes.
All of the above, except the last, effectively cause your device to refresh its IP and DNS settings.
Hopefully one of the above will resolve the problem. It’s also worth noting that you might be getting tight on the necessary free space for a major version-update.