Power off the computer and unplug it
Remove the CPU cooler
Wipe off excess thermal paste with a dry lint-free cloth or paper towel
Dampen a lint-free cloth or cotton swab with 90%+ isopropyl alcohol
Gently clean the CPU heat spreader until all paste is removed
Clean the cooler base the same way
Use fresh swabs or cloth sections as needed
Let all surfaces dry completely
Reapply new thermal paste before reinstalling the cooler
Reattach the cooler securely
Check that no paste is left on nearby components