The process of renewing a certificate is more like generating a new certificate except that you need to generate a new "renewal CSR" and not use your old one.
You will be allowed to renew a certificate only within 30 days of its expiration. When certificate is close to expiration (30 days) you have an invoice generated for renewal certificate.
After invoice is paid, your certificate's status changes to 'Awaiting configuration' and you are to activate it. Please click here to find more information on how to generate a renewal CSR.
Continue with the activation process and approve the certificate. You will then be issued the renewed certificate.
NOTE:
(1) Please ensure that you generate and use the "renewal CSR". If you put in a regular CSR (i.e. the CSR generated during initial certificate activation), you are going to lose days or will get an error when trying to validate the CSR.
(2) If your certificate has already expired, you will not see a "Renew" link for the certificate in your account. In such a case, you will need to request a refund for the renewal SSL and purchase a new SSL and activate it.