Apple App Store
Apple do not provide developers a method of offering refunds.
If you want to request a refund you will need to contact Apple directly. Please follow the instructions Apple have provided for claiming a refund.
It will then be down to Apple's discretion as to whether they issue the refund or not.
Google Play Store
Google do provide developers a method of offering refunds.
If you want to request a refund for your Google Play Store purchase please provide a team member your order number. This will have been emailed to you or can be found in your purchase history. Order numbers begin with GPA.