I got my Lumia 640 dual SIM phone 10 days back which has W8.1 DENIM uploaded. From past 3 days I am facing severe heating problem at top of the phone, this only happens when I am using GPS, playing games, using Facebook, WhatsApp, browsing in Opera & uc browser, rest of time it won't get heated up. How can I solve this issue?
Playing games uses a lot of processing power, which in turn generates excess heat; Similarly, some webpages use a lot of client side scripting that can use a lot of processing power.
From what you've said, it may well be the case that you're seeing a "normal" amount of heat generated. If you're concerned, you could pop by you local friendly service centre, and they should be able to confirm that it is running within expected tolerances.