Once logged into you account, you can simply cancel your active subscriptions by clicking on your name in the top right corner, and selecting My Subscriptions from the menu.
In the following page, click on the subscription you want to cancel, then simply click cancel.
Once cancelled, the subscription will expire, and you will not be charged for the next billing cycle. You can still book and attend classes for the subscription period you have paid for.
Lear more about cancellations and refunds here.