Make sure you're logged into your account.
PC/Desktop: click on the 'Membership' menu option.
Smartphone: click on the hamburger menu (top-right), then click on the 'Membership' menu option
You will now see the various membership plans, and the one you're currently subscribed to.
On the plan you're currently subscribed to, you should see a red 'Cancel' button.
Click on that, and then 'Yes' to confirm.
Please do continue to enjoy the benefits of a free basic account!