From their terms:
You may cancel your subscription for the Services by logging in and going to the My Account page and clicking on the “Cancel Subscription” button or sending an e-mail to customer services at the following e-mail address: firstname.lastname@example.org.
However, for most services, the “Cancel Subscription” button is not available at all. So use the email address if you can’t find it. Note that no refunding will take place!