To cancel your premium membership, please go to Settings-->Huckleberry Subscription-->Manage Subscription. This will bring you to the screen where you can change your membership status.
If you purchased through iTunes and the shortcut above does not bring you to the right place, it is likely because you are not signed into your iTunes account. Here are instructions on how to directly cancel your membership through iTunes:
Please get in touch at firstname.lastname@example.org with any questions, and we’ll be happy to assist you.