For parents:
To change your subscription level you must cancel your existing subscription, and re-subscribe. This process can be started by cancelling your subscription via the “Manage Payment Account” link on the Dashboard Account Settings page. To find this log in to the Dashboard, and click “Account”. After cancelling, you can purchase a new subscription as normal.
For students:
Students can upgrade by going to the Settings tab, and tapping “Upgrade”. The App Store / Play Store handles the upgrade and proration process.