If Certbot is installed in Debian, renewal of certificates should be already automated and there is nothing else to do:
The Debian Certbot package installs a systemd timer which runs every 12 hours, calling a Certbot service. The Certbot service then executes the Certbot command for renewing expiring certificates.
To make sure the certbot timer is running, check the list of active timers:
$ systemctl list-timers
Or look at the status of the Certbot timer itself:
$ systemctl status certbot.timer
To see what exactly the Certbot service does:
$ less /usr/lib/systemd/system/certbot.service
More: Certbot - User Guide