You should do it step by step, first php8.0 then php8.1 etc.
Since you are on Ubuntu 20.04 (focal), you can use the → php-updater script ← for that. It is made to make life easy 
But I would seriously recommend that you switch to a newer Ubuntu first, focal is really no longer up to date in many ways. The extended PHP support for focal ends in 10 months. The current LTS version is 24.04 and will be supported until 06.2029, so almost 5 years from now.
Much and good luck,
ernolf