How do I upgrade from PHP 7.0 to 7.3 on google cloud platform?

I am running the WordPress multisite click to deploy on the compute engine.

Operating system
    Debian 9.8 
Package contents
        Apache 2.4.25
        Google-Fluentd 1.6.9
        MySQL-Client 5.7.26
        MySQL-Server 5.7.26
        PHP 7.0.33
            Stackdriver-Agent 5.5.2
            WP-CLI 2.2.0
            WordPress 5.1.1
            phpMyAdmin 4.6.6

I would like to upgrade PHP to 7.3

Answer

You can use a third-party repository to install php7.3

apt-get install ca-certificates apt-transport-https 
wget -q https://packages.sury.org/php/apt.gpg -O- | sudo apt-key add -
echo "deb https://packages.sury.org/php/ stretch main" | sudo tee /etc/apt/sources.list.d/php.list
apt-get update
apt-get install php7.3