If you are interested in upgrading your account from a monthly account to an annual account, you will need to cancel your current upgrade before you can switch to annual.
You may then hover over of "Account", then select "Payment Method". On the next page, you will see a green "Premium Services" bar. Directly under the bar, you will see a gray "Cancel Upgrade button".
This will delete your credit card information and you will be able to select the annual fee when you restart the upgrade.
Upgrade the account, this time choose Annual instead of Monthly and re-enter your payment information.