Change the default PHP version from multiple PHP version (Version 7.0 to Version 7.1)

Disable the current php version
$ sudo a2dismod php7.0

Restart the apache
$ sudo systemctl restart apache2

Enable the expect php version
$ sudo a2enmod php7.1

** If problem arise then the following two line

$ sudo apt purge libapache2-mod-php7.1 libapache2-mod-php

$ sudo apt install libapache2-mod-php7.1 libapache2-mod-php

Enable the expect php version

$ sudo a2enmod php7.1

Restart the apache

$ sudo systemctl restart apache2

Check the php version

$ php -v

Finally set the php version

$ sudo update-alternatives --set php /usr/bin/php7.1

Check the php version

$ php -v

Restart the apache2

$ sudo systemctl restart apache2