Only the primary account owner will be able to make adjustments to the type of membership they are subscribed to. To manage your subscription, click "⚙️ Settings" on the left side navigation bar, then click "Billing & Subscriptions".

From here you will see the Orgs that you manage. Locate the membership you want to make changes to, and select "Manage Subscription". This area provides you access to all the changes you can make on your end.