Generally downloadable products are non-refundable, however in some special circumstances there can be an exception. However, all transactions are handled by Apple iTunes and Google and they handle the actual refund requests.
For Apple (iOS) devices:
Visit Apple support and follow the steps included on their page.
For Google (Android) devices:
Visit Google support and use the ‘Contact us’ button to reach their Customer Support.
For Amazon devices:
Visit Amazon support and follow the steps or contact their Customer Support for more assistance.