How do I delete data on my iPad Mini 3?
If you are on iOS 12, go to Settings -> [your name] -> iCloud -> Manage Storage

iCloud page:



Then click on your app in the list.

Then look for your games data in the list.

Then click on it and click delete data:



This should clear all backed up data for the app. Then reinstall the app.

Alternatively, you can try and look for a setting in the game to reset your progress.