Only the Account Owner for the gym will be able to change the subscription. The "Account Owner" title is given to the person who initiated the sign-up process with Beyond The Whiteboard. This can be done at any time. Simply click on your gym's icon in the top-right of the header and then on the "subscription" link from the drop-down menu that appears.
From the subscription page we'll show you how many members you currently have and how many more you'll be able to add on your current subscription. Click on the "current subscription" link to upgrade/downgrade your subscription.