Unfortunately, certbot currently does not have an easy way to modify (i.e., add/remove) hostnames from an existing certificate. You'll need to "re-issue" the already existing certificate with the same command you've used previously, but now with the modified list of hostnames using -d. I.e., to add a third hostname, you'd use the existing two -d options and add a third -d option. And if you'd want to remove one of the hostnames, you'd use one -d option less.
To prevent certbot from adding separate certificates (so you'd end up with your old, unused certificate and the new one), I recommend to add the option --cert-name followed by the certificate name you can find when running the command certbot certificates.
Also, your certbot is very old.. I recommend updating to a more recent version.