Start on your account's Home page.

Find the Account Overview section. There should be a link or button that says My Billing – click it.

This will take you to the Current Payment Methods page, where you'll see your current plan. Click the CHANGE PLAN button.

You'll now see the Plan Details page, which compares the features and prices of the Premium, Essentials, and Paygo plans.

If you want to upgrade, click the Upgrade button for the plan you want.