Once the heatsink has been removed from the processor, the original thermal paste should never be re-used, as all of its thermal properties will have been destroyed. The processor and heatsink should be cleaned and new thermal paste applied to the processor for correct operation. To protect the processor from pin damage, please remove the thermal compound while the processor is still mounted in its socket. Then, using a paper towel or tissue paper, apply a mopping action on both the processor and heatsink to wipe off all the old grease. If the thermal grease is dry, please use an alcohol solvent to help remove it. AMD has created a video that explains how to replace a processor. This video also covers the procedure of cleaning the processor and heatsink.