kunthakenthe :
use rubbing alcholol to clean it.
Actually, You're going to want to use Isopropyl Alcohol to clean it. (99% or 100% if you can find it) Conventional Rubbing alcohol contains ~30% water content.. which you don't want in contact with your motherboard or CPU
The higher the percentage the quicker it evaporates/drys which is helpful while working in your case. When you clean it, don't use rough cloths or Q-tips (as they leave remains behind), Try using a coffee filter over your finger tip to remove and then re-apply the paste, or simply use the Pea method)