Switching from Monthly to Yearly actually works out cheaper! You get 2 months on us, so you actually only pay 10 months!
To switch from Monthly to Yearly payments, head to your Account Settings in the top right corner of your account.
This will take you to the Billing & Usage page. On the right-hand side, click on the box that says "Switch to yearly". This will open up a new window showing the cost of your yearly subscription, with 2 months hosting for free.
If you have already paid for a Month, the rest of the month you're in will be applied as credit. This gets discounted right away from your total price. If you're happy with the amount, click Proceed To Checkout.
Note: Once you complete checkout, you will be charged immediately for this upgrade.
Then just complete the checkout procedure and you're all done and paid for the year!