You must submit a new CSR and use their renewal issue link, and it will add the remaining time to the new certificate.
Steps:

  1. Generate new CSR (see Cornerstone Quick Start Guide http://southrivertechnologies.com/docs/cornerstone/Cornerstone_FTPS_Public_Key_%20Quick_Start_Master.pdf).
  2. Submit new CSR as Renewal, then the remaining time will be added to new cert. 

NOTE: You can obtain a free reissue using the automated reissuance page below: 
http://www.geotrust.com/ssl/renew-ssl-certificates.html.