Do I need to apply thermal paste to heatsink on laptop?

If you are going to tear your laptop down sufficiently to where you can get to the CPU, then regardless of whether it is running hot or not, you should change your thermal paste to a high-quality thermal paste.

If your laptop is running really hot, then you should definitely change the thermal paste, which means that you will need to tear it down sufficiently to where you can get to the CPU.

It is a real chore to tear down a laptop; so don't tear it down unless you have a good reason to do so. It is very easy to break a tiny, essential part, so be very careful when you are tearing it down.

It is strongly advised that you wear an anti-static wrist strap (Anti-Static Wrist Strap) when you are working on your laptop, so that you don't damage a part due to static electricity.

Use either Arctic Silver ArctiClean (Arctic Silver ArctiClean) or high-grade isopropyl alcohol (91%) and q-tips to clean the surfaces of the CPU and the heat sink (after you remove the heat sink from the CPU); then apply Arctic Silver 5 Thermal Paste ([연계 제품이 누락 또는 비활성화 됨: IF179-010]) to almost cover the face of the CPU. The idea is that when you push the heat sink onto the CPU, it will spread the thermal past, so you want it to end up covering the entire face of the CPU. (If any excess comes out, clean it with a q-tip.)

I will echo the recommendation to apply new thermal compound after removing the old. I will add however, that it is critical that you do NOT have any air bubbles in it. Air is a very good insulator and you don't want insulation here. You want good heat conduction to the heat sink. A little too much compound is better than not enough. Just don't go overboard with the amount. Spread a thin layer without any bubbles across the entire CPU surface. If it spills over the side, again don't go crazy, but it won't be a problem. This compound is a heat conductor not a conductor of electricity.

Dan