I want to downgrade PHP version

You can downgrade the PHP version using the command in the terminal.
sudo apt-get remove -y --purge php7.0 *
sudo add-apt-repository --remove ppa: ondrej / php
sudo apt-get update
sudo apt-get install php5-fpm php5-mysql
If you execute the above command in order, you can use php5.
Libraries like GD and MariaDB can also be used after entering the commands in the terminal.