Certbot add www domain to existing domain certificate

I have installed an SSL certificate from Let's Encrypt with Certbot on my Apache server with Debian 8 following this tutorial from Let's Encrypt's own documentation:

$ certbot --apache

You need to specify the domains where you want to install the certificates for, but I only added the example.com domain. Now I want to add the www.example.com, but cannot find how to do this.