To cancel, click on the "Account & Billing" tab. Once you are on the Account & Billing page, scroll to the link just below the "Billing Status" box that says "Want to cancel your account?". Select the link to open the confirmation screen and select the button that says "Yes, Cancel My Account". If you change your mind, select the link that says "No! Don't Cancel My Account"
Note: There is no undo for cancellation. Once your account is cancelled, all your information will be immediately and permanently deleted.