I can’t install the new iOS after downloading it

What happens when you try? Error message(s)?

Update your iOS device.        

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      

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

See if anything here helps.

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      

Gdgh

I have just downloaded the update on both my iPhone and iPad and although the install button appears, you cannot press on it.