A renewal reminder is sent by email 60, 30, 15, 7, and 1 day before the expiration of your certificate. You can also see the expiration date on your SSL interface.
To renew your certificate you must go through the same validation process as for your previous certificate. This is a necessary security measure that can not be bypassed.
Once your certificate has been renewed, the new expiration date is calculated based on the subscribed time.
At this point you must generate a new CSR and then, re-install your certificate. It is not possible to keep the former CSR to launch the renewal of your certificate.