Software update is stuck

See If your iPhone, iPad or iPod touch won’t update https://support.apple.com/en-gb/HT201435

Also, see If you see an error when you update or restore your iPhone, iPad, or iPod https://support.apple.com/en-us/HT201210  and

iOS update and restore errors https://support.apple.com/en-us/HT204770

More information.

Get help with over-the-air iOS updates          

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