A renewal will not extend your existing certificate expiration date, it will result in a brand new certificate that must be installed in place of the existing one. Both certificate and private key must be replaced on the server. There is no way to extend the expiration of an existing certificate
If you have discovered the SSL certificate expired and you did not get the renewal email, we can still help you if you have a Comodo product. We have an automated system that will begin to contact all of our customers at about 60 days before the expiration date with information on how to renew and a convenient link.
Sometimes, due to email spam settings or other issues, these renewals are missed. If you have changed emails, you can still complete the renewals by logging into your account through the site and following the instructions.
To renew after discovering an SSL certificate expired, just generate a new CSR and provide your password. If you do not have your password, contact our staff via email and we can help to provide the information needed. You will need to use the email listed on the expired documentation.
Remember, when the SSL certificate expired it did so because the dates to cease functioning are coded into the SSL certificate. This means when you obtain the new or renewed SSL certificate you will need to install this in the system in order to activate.