I downloaded the app on my iPhone 6s and installed it there. But a few days ago I upgrade to iPhone 7, I would like to keep the saves and preferences of the game on my new iPhone. Any way can do that?
Back up your phone to a computer then when you use the new phone sync it to the computer and use that backup. Everything should remain the same
You can also back up to iCloud -- as long as you sign in with the same Apple ID on the new phone you'll be all set.
Be aware that this will not back up the apps themselves. The last stage of the restore process is downloading all of your apps from the store. If you have any apps that are no longer in the store they will be lost unless you have them backed up on a computer and reinstall them from there after the restore.
Move to iOS app or restore from iTunes backup on iPhone 7. The app is published by Apple official a few months ago and it is the recommended way to transfer data from iPhone to another iPhone. However, it takes much more time than other apps.
