From php5.6 to php7.0:

Apache:

sudo a2dismod php5.6 ; sudo a2enmod php7.0 ; sudo service apache2 restart

CLI:

sudo update-alternatives --set php /usr/bin/php7.0

From php7.0 to php5.6:

Apache:

sudo a2dismod php7.0 ; sudo a2enmod php5.6 ; sudo service apache2 restart

CLI:

sudo update-alternatives --set php /usr/bin/php5.6