First of all, please make sure that you are logged in with the app store account that was used to make the purchase. Please follow the steps below to re-download the app/game.
For iOS
1. Open the App Store
2. Tap the sign-in button and select “Purchased”
3. Open the “Not on this iPhone/iPad” tab
4. Tap on the download button next to the app you want to download
For more information, visit the Apple Support.
For Android
1. Open the Google Play Store
2. Open the side menu and select “My apps & games”
3. Open the “Library” tab
4. Tap “Install/Enable” next to the app you want to download
For more information, visit the Google Play Help.