Step 1: Go to the Billing area
Step 2: Scroll to the middle of the page and click "Manage Subscription".
Step 3: Select the dropdown showing your current plan, then choose the new plan you'd like from the list
Step 4: Once you've chosen your new plan, click the Update Subscription button
In case you're wondering
Any unused funds from your current plan will be applied as credit to your new plan, so don't worry about losing what you've already paid for :)