First of all, please make sure that you are logged in to the right account (the one you did the purchase with).


For iOS:
1. Open the App Store
2. Choose Updates -> Purchased -> Not on this device
3. Choose the app you want to reinstall
For further information follow this link.


For Android:
1. Open the Google Play Store
2. Tap Menu -> My apps & games -> Library/Archive/All
3. Choose the app you want to reinstall
For further information follow this link.