Power off the device and unplug it
Remove the heatsink or cooler
Use a dry lint-free cloth or paper towel to wipe off the bulk of the thermal grease
Dampen a lint-free cloth or cotton swab with isopropyl alcohol
Gently clean the remaining grease from the CPU, GPU, or heatsink surface
Repeat with fresh alcohol and clean swabs until the surface is clean
Let all surfaces dry completely
Apply new thermal grease if needed
Reinstall the heatsink or cooler securely