I have been overcharged in a game app

subscriptions purchased or applications downloaded from the App Store are managed by Apple. This means that these subscriptions or purchases and any other payment configuration can only be viewed and configured on your Apple account.

If you have other questions or concerns about canceling the subscription, you can refer to this link: View or cancel your subscriptions - Apple Support

If you need further assistance to cancel the subscription, please contact Apple Support.

You can contact Apple Support to request a refund. In most cases, they will grant it without bigger problems.