Most certificate types allow for renewal within 30 days of having expired. 

To renew within this window, open the Certificate Management Center page, and choose 'Cancel' when it asks to choose your certificate.  It will then allow you to log in with your Account Number and DST Passphrase. 

Once inside CMC, there is a check-box that states "All my certificates have expired, but I would still like to request a renewal".  Check that, and click 'Continue', and the system will walk you through the renewal process.