Go to More > Premium Features screen, scroll to the bottom of the screen and tap on Restore
If that doesn't work, try the following steps.
- Go to iOS Settings > iTunes & App Stores and tap on your Apple ID, logout and log back in with the same Apple ID you used to purchase the app.
- Uninstall the app, reboot your phone and and reinstall the app from the App Store.
If you are still experiencing problems, send a screenshot of your proof of purchase to firstname.lastname@example.org
. You can find your proof of purchase by following the instructions on Apple Support