You can downgrade an account in the Membership tab of "My Settings. From there, you can find a record of all your Payment Transactions as well as your current points balance and your membership status. If you have a free promotion, you may end your free promotion at any time and shift to a different account level. 

While logged in, click the Menu button in the top right corner of the screen and then select "My Settings" from the menu.

Once you enter your settings screen, you will see a new menu below the main blue navigation bar. Select "Membership." You will see your membership information and your points as well as your history of transactions. To change your membership, click the highlighted (tap to change) text. 

You will be taken to a new page where you can see your membership status and a breakdown of all the membership tiers. To downgrade to a lower tier, press the "downgrade" button in the column of your desired membership.