All payments are handled by Apple through your App Store Account. You'll need to reach out to Apple to request a refund via their problem page or using the 'report a problem' link in the email you will have received from Apple.
We recommend asking for a refund as soon as possible, as it's more likely to get a refund for a payment that has just been completed.
If you purchased through our website then that payment is handled by Stripe. Simply get in touch with our customer support team and we'll be happy to issue a refund.