Super simple-first login like normal. Then click your account link from the upper right. Select the 'manage subscription' button.
Then select the 'CHANGE PLAN' button. Select the yearly plan and follow the steps. The system will automatically cancel your monthly and it will add any remaining dollar amount from your monthly and deduct that from your yearly. ( prorate) Cool right?
Your yearly subscription will start immediately and you will be billed on an annual bases instead of monthly!