Answers for "how to change my php version to 7.4 on ubuntu"

ubuntu update php 7.4 to 8

sudo apt-get purge php7.*
# recommended 
sudo apt-get autoclean
sudo apt-get autoremove
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
# end recommended 
sudo apt-get install php8.0

change php version in ubuntu

sudo update-alternatives --config php
sudo update-alternatives --config phar
sudo update-alternatives --config phar.phar