On your web browser, go to www.facer.io.
Login to your Facer account (the same one used to purchase Facer Premium).
Navigate to your profile and click the 'pencil' icon to edit your profile.
On the edit profile page, click the 'pencil' icon next to where it says 'Subscribed' in the Facer Premium section at the top.
After successful cancelation, your subscription will lapse at the end of the current period and will not be renewed.