If you want to upgrade your subscription, such as switching to Pro Yearly in order to download HD videos, then you can do this from https://cgcookie.com/register
First login to your account then visit the register link above and choose the plan you'd like to upgrade to (typically this will be Pro Monthly or Pro Yearly). After you choose a plan you should see a breakdown of the changes, including the total you'll be charged today, the proration credit from unused time on your current subscription, and how much you'll be charged upon renewal.
Here's an example of a breakdown when upgrading fro Pro Monthly to Pro Yearly:
To complete the upgrade click the Submit Payment button and then enter your card details. You will then be charged for the amount shown under Total Today and switched to the chosen plan!