Icloud and IOS 14.0

Hi. My Dad use iphone 6 Pro Max. And he always forget his ICloud with both username and password. So I note his Icloud acc on his phone. Unfortunately, his phone upgrade to IOS 14.0 automatically and they need sign in his ICloud acc. But we forget it. Now he do nothing with his phone. Can you please stop upgrade? If you succeed, I will upgrade for him later when I can open his phone. Thank you so much