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 ([product|IF145-071]) when you are working on your laptop, so that you don't damage a part due to static electricity.
Use either Arctic Silver ArctiClean ([product|IF179-023]) 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 ([product|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.)