Your paid subscription can be canceled via the 'Profile settings' - 'Billing' tab anytime.
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':
You will retain paid features until the end of your current billing cycle. Your subscription status, displayed below the plan name, will show the cancellation date. If you prefer to cancel your subscription immediately, refresh the page and click the 'Cancel now' 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.