This page provides instructions on how to upgrade Ubuntu from PHP7 to PHP8. Much of the information for this page came from this link. https://devanswe.rs/how-to-upgrade-from-php-7-x-to-php-8-on-ubuntu-apache/
Check Ubuntu Version
Ensure your Ubuntu is at least 20.04. To do this run this command in Ubuntu.
lsb_release -a
Remove PHP7
Once you have confirmed that you are on the right version of Ubuntu remove your current version of PHP by running this command.
sudo apt-get purge php7.*
If prompted press yes.
To remove remaining elements of PHP7 run these two following commands.
sudo apt-get autoclean
sudo apt-get autoremove
Press yes if prompted.
Install PHP8
sudo apt-get install php8.1 php8.1-cli php8.1-mbstring unzip php8.1-gd php8.1-zip php8.1-xml php8.1-curl php8.1-imagick
sudo apt-get update
sudo add-apt-repository ppa:ondrej/php
sudo apt -y install software-properties-common
sudo apt-get update
sudo apt-get update
curl --version
php --version
And you should be good.