To upgrade your subscription, go to the Billing tab within the members area and hit the Change link next to your product. You will be prompted with a screen consisting of a variety of options for upgrade. Select the option that fits your needs and hit the Upgrade Subscription button.
To downgrade your subscription, locate your subscription on the home page of the members area. Next, you'll need to cancel your existing subsription - your service will not be interrupted as the subscription will remain active until the end of the billing period. Then hit the Add A New Subscription button next to Active Subscriptions and subscribe to the plan that fits your needs.