//instalar alguma versao do PHP
$ sudo apt install php5.6 [PHP 5.6]
$ sudo apt install php7.4 [PHP 7.4]
$ sudo apt install php8.0 [PHP 8.0]

//desativar a versão
sudo a2dismod php8.0

//habilitar uma nova versão
sudo a2enmod php5.6

//reniiciar o apache
sudo service apache2 restart

//setar versão
sudo update-alternatives –set php /usr/bin/php5.6
sudo update-alternatives –set phar /usr/bin/phar5.6
sudo update-alternatives –set phar.phar /usr/bin/phar.phar5.6

retirado de: https://www.tecmint.com/install-different-php-versions-in-ubuntu/

retirado tbm de: https://stackoverflow.com/questions/42619312/switch-php-versions-on-commandline-ubuntu-16-04