See if anything here helps.
Get help with over-the-air iOS updates
Update – Can’t or can’t restore
Have you tried using iTunes on a computer or Finder if running Catalina to do the update?
Update software on iOS devices in iTunes on Mac
If you see an error when you update or restore your iPhone, iPad, or iPod
Do you have access to another WiFi network where you can try? That has worked for several people.
Try a restart.
Restart your iPhone, iPad, or iPod touch
Try a forced restart.