Download Good Guardians from the galaxy store, then downliad the thermal guardian module. You can check it when your phone is heating up to see what's going on when it's heating and you can possibly come up with a solution. Like if jt days high cpu usage and then list the apps causing it, you can shut those apps down or stop background activity.
What are you doing during the heat? The processor will heat up if you are using a lot of resources. That's pretty normal. You can leave the case on. The phone will automatically throttle and adjust to give you the best experience with the case on. Let the phone Donald of the work. As the author says above, you can force the phone to throttle (slow down) at lower temperatures. This is basically what the companies do when customers say the phones are too hot. They release software updates to adjust the phone throttling.
