What did you do that caused the Samsung Galaxy Note 8 or Note 9 to overheat? Did you leave it somewhere very hot, under the direct sunlight? If you did, suffices to turn it off and let it sit in a dark, cool place, away from the direct sunlight, and it should recover just fine.
If you have used it intensively to play games or to listen to music with the navigation turned on or anything else that has been seriously consuming the resources of the device, again, you shut everything off. Check it after a couple of minutes and you should already notice an improvement.
But if you did nothing from the entire above yet the Samsung Galaxy Note 8 or Note 9 continues to overheat, there’s probably a third-party app that decided to mess with your smartphone. If that’s the case, you need to be sure and you can only do that from the Safe Mode.
Reboot the Samsung Note 8 or Note 9 into the Safe Mode, where no third-party app runs. If it easily cools down, you have your answer. You know it’s a third-party app to blame, but you don’t know which one? You can test them one by one, manually uninstalling them and monitoring if the device gets hot or not. Or you can try the factory reset and start reconfiguring the phone and installing only the third-party apps that you really need.
If you go for our last suggestion on how to handle a Galaxy Note 8 or Note 9 that overheats, don’t forget to wipe the cache partition as well, before the reset. You might have the surprise to discover how the phone actually needs no reset and the wiped cache alone was enough to stop the overheating problem.
Turn off the device, then press and hold the Volume Up, Home, and Power keys. When you see the Samsung logo and that blue recovery label from above the logo, you’re in the Recovery Mode. Scroll down, with the Volume Down key, to the Wipe Cache Partition. Turn it on with the Power key and reboot the Samsung Galaxy Note 8 or Note 9 when you finish.