SSLs cannot be upgraded. However, it is easy to transition to a Premium Extended Validation (EV) SSL. Purchase a new EV SSL in the same customer account as your existing SSL, and request it for the same common name.
Your current SSL remains active while your EV SSL request is vetted. When you receive the new certificate, simply install it over your existing certificate.
Note: If the common name is in use in a different customer account, you cannot request a new certificate using that name until you revoke the original certificate from the original account.