If you purchased on Google Play and it has been less than 48 hours, you can request a refund directly from Google Play. If it has been less than 2 hours since purchase, simply uninstalling the app should initiate the refund process. For any other case, contact me with your purchasing email address and I can refund your purchase.
If you purchased on Amazon, their policy states:
Apps and In-App Products purchased from the Amazon Appstore may be eligible for a refund within 90 days of purchase, depending on the circumstances of your order. To request a refund, visit Your Orders and select Return for Refund next to that order. If you do not see a Return for Refund button, you are not eligible for a refund for that order.
If the above does not work, you’ll need to contact Amazon support to pursue a refund. They do not provide developers any insight or control into their orders and payment system, so I have no ability to refund Amazon app purchases myself.