# php -v (default PHP version)

# From PHP 8.0 to PHP 7.4:
sudo a2dismod php8.0
sudo a2enmod php7.4
sudo update-alternatives --set php /usr/bin/php7.4
sudo service apache2 restart

# From PHP 7.4 to PHP 8.0:
sudo a2dismod php7.4
sudo a2enmod php8.0
sudo update-alternatives --set php /usr/bin/php8.0
sudo service apache2 restart