Over heating iPhone

If you don't get a warning then it is not offically overheating 

Please see If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support

Overheating during In-App game playing could be caused by things other than the game you are playing when it heats up. You need to remove apps from your phone that you do not use. You can check these apps under battery usage in your settings. Make sure your phone is up to date as well which can be done at settings>general>software update on iPhones. You can also dump your iPhone's stored data by caching it which is done at settings>scroll down to safari and select> scroll down to clear website data and history. Restart your phone once you do this. If it still overheats, you need to check your screen brightness settings for correction. Don’t leave your phone in extreme temps either. If all else fails contact apple support who is able to remote into your device’s software and diagnose the issue for you. You can do this by calling 1‑800‑APLCARE

If absolutely none of these work, do a backup of your phone under your ID in settings. Once backed up>sign out of your apple's iPhone in settings> then select general>transfer or reset iphone>once it reboots you will have to go through the steps of setting up a new iPhone and then signing back into your account with the latest backup>this should be the last thing you do as you can lose data such as text, images, etc and if you did not remove the app draining your battery, it may not help because once you are signed back into your account on your iPhone the app that was consuming so much battery will do it again. I suggest this last step when all else fails but it is possible that you lose data so avoid it if possible. Apple care should be the alternative in this scenario when you are unable to stop overheating. Good luck.

The OP may not be in USA

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or phone numbers

Click here Contact Apple for support and service - Apple Support