To downgrade your subscription to Free and cancel your paid subscription, click on the "Free" option and then click "Change plan" on your Subscriptions page here:
https://leanpub.com/author_dashboard/subscriptions
You can also access this page from the main navigation window at the top right:
On the Subscriptions page, click on the Free option:
Next, scroll down and click "Change plan":
Finally, accept the "Are you sure you want to switch plans?" prompt by clicking "OK":
That's it!