Follow following steps
1. sudo add-apt-repository ppa:ondrej/php
2. sudo apt-get update
3. sudo apt-get install php7.1
4. sudo apt-get install php7.1-cli php7.1-common php7.1-json php7.1-opcache php7.1-mysql php7.1-mbstring php7.1-mcrypt php7.1-zip php7.1-fpm
5. sudo a2dismod php7.2
6. sudo a2enmod php7.1
7. systemctl restart apache2
8. sudo rm /usr/bin/php
9. sudo ln -s /usr/bin/php7.1 /usr/bin/php
