If you are currently on a monthly-renewing subscription, you may absolutely upgrade your account at any time.
Click on the "Settings" Gear icon, then select the "Parent" icon when prompted.
- On the Parents Page, select the "My Account" panel
- From the My Account Panel, select the "Membership" panel
- From the Membership panel, click the button labeled "Upgrade"
From the Upgrade area, you will be automatically billed at the time you make your selection, and the additional months are added to your existing subscription.