First off, know that your first month of Duckr usage is absolutely free, and no strings attached. If you still would like to cancel your subscription, or prevent it from renewing, Apple has posted some steps on how you can do this.
You can find the article here.
Note that the billing for Duckr, as well as all of the subscription payments, are handled by Apple and not the team at Duckr. We are unable to process any refunds or change your own personal billing information in any way - it is entirely controlled by you, and Apple.
The Duckr Team