Thermal paste fills microscopic gaps between the component (CPU and GPU) and heatsink. It helps transfer the heat from the CPU or GPU to the heatsink. But overtime the thermal paste dries up and become ineffective so it is a good practice to change or re-apply the thermal paste. Dust also affects the temperature of the CPU and GPU by blocking the air from the fan to the heatsink, so regular cleaning helps your card to perform like brand new.
What You Need
Small paint brush
1. Remove the components that blocks your way to remove the CPU heatsink (graphics card, RAM modules). Disconnect power cables for CPU and motherboard. Also disconnect the CPU fan header from the motherboard.
2. Remove the CPU heatsink. How to remove the CPU heatsink varies on what CPU model you have. Look for the CPU manual for specific instructions.
3. Remove the old thermal paste by wiping it in circular motion inward using the cloth. Put isopropyl alcohol on the cloth if the old thermal paste is dried up and hard to wipe. Do it both on CPU and heatsink.