Interactive switching mode
sudo update-alternatives --config php
Manual Switching
From PHP 5.6 => PHP 7.x Default PHP 5.6 is set on your system and you need to switch to PHP 7.x.
Apache:
$ sudo a2dismod php5.6
$ sudo a2enmod php7.1
$ sudo service apache2 restart
Command Line:
$ sudo update-alternatives --set php /usr/bin/php7.1