Well there are quite a few reasons why your processor might be overheating. It could be lack of thermal paste, insufficient cooling , improper airflow, bottle-necking due to mismatching components and so on. These are the only things i can think of which will help you with your problem. Try them out and hopefully you’ll get results!
I have a Dell laptop (Inspiron 3521). I bought it a few years ago and until now  I didn't do any services or cleanings (I mean disassemble the laptop and cleaning). I recently noticed that the CPU temperature is going high like 80c when working and when idling it is between 59c - 69c (mostly 65c - 70c). I am using Windows 10. So is it an issue of BIOS or any other issue? Or should I service the laptop now? Thanks in advance.
Laptop CPU heating up to 80°C