To do that, just go to the "Account" menu in the upper right-hand corner and click the "Billing" tab. Now, simply remove your credit card details by clicking the button marked with a red arrow on the screenshot below and...it's simple as that.
Your account will remain active until the end of the current billing period. If we aren't able to charge your card again, your account will expire and we'll send you a goodbye message. And then we'll cry ;-(
If you wish to cancel your account while you're still on trial, you don't have to do anything. When your 14-day trial expires, your account will be deactivated automatically. And then we'll send you a goodbye message and cry twice as much.