‍In the App Store: Go to reportaproblem.apple.com.  Sign in with your Apple ID and password. If you see the "Report" or "Report a Problem" button next to the item that you want to request a refund for, click it. Follow the instructions on the page to choose the reason why you want a refund and submit your request. 

In Google Play Store: You can request a refund on the Google Play Help website.