If Certbot is installed, renewal of certificates should be already automated and there is no need to do anything else:
The Debian Certbot package installs a systemd timer which runs every 12 hours and calls 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:
Or look at the status of the Certbot timer itself:
systemctl status certbot.timer
To see what exactly the Certbot service does:
More: Certbot – User Guide