You can restore your previous purchases by:
Android
In-app purchases are linked to the Google account where they were originally made so make sure you're currently using the same account you used for the initial purchase and then go to the Tools tab and tap Restore Purchases on the settings page of the app.
iOS
In-app purchases are linked to the Apple iTunes ID where they were originally made so make sure you're currently using the same account you used for the initial purchase and then go to the Tools tab and tap Restore Purchases on the settings page of the app.
On iOS we have three different apps (OutDoors GB, OutDoors GPS, OutDoors France) and Apple will only restore the purchase if you tap restore in the same app as your original purchase. To determine which app the purchase was made from simply go to the App Store, tap on your account photo in the top right of the Today tab and select Manage Subscriptions. Then simply open the correct app and tap Restore Purchases.