When you are at that site, just click on "Renew Credential" and enter your own information at the prompt.
It is easy -- and credentials are processed in 10 working days.
The issuance date for the renewed credential will be the same as the expiration date of the previous credential. You do not get a different expiration date by renewing early.
Don't wait -- check your information and renew on time, or early. You can renew your credential up to 12 months in advance of the expiration date. Do not let your credential lapse -- doing so will jeopardize your pay.