Your paid subscription can be canceled via 'Profile settings' - 'Billing' tab at any time.
Note that once the subscription has been canceled, your account will become free immediately.
Click the cog icon ('Profile settings') in the top right corner then select the 'Billing' tab. Next, click the 'Cancel' link next to the name of your plan, select the reason for canceling your subscription and click the blue 'Cancel subscription button':
Note that it is not possible to manage your billing information or cancel your subscription via the iOS or Android apps - please use the web version instead.