You can change your plan at any time by going to:
Subscription & Billing or Manage Billing
Click Update Plan
Finally, you'll see the total amount before you Confirm.
You don't need to cancel your monthly plan to upgrade. When you switch to the annual plan, you'll only be charged for the prorated difference of what you already paid for. Even if you've already been charged for this month, the amount you paid will be discounted from the annual plan when you switch.