If you have paid your Supermetrics for Google Drive Pro license with a credit card, you can do the upgrade from your Account Management page by clicking "Upgrade to Super Pro".

If you have used PayPal, or paid by invoice, please send us a private ticket requesting the upgrade.

In both cases, your remaining license value is calculated to match Super Pro subscription level and you don't have to pay extra at this point. (If you have a license that is not expired)