Unscrew the 5 screws holding the CPU heatsink. Lift it up, gently.

Clean the old thermal paste both from the cooler and the processors. It is best to use isopropyl alcohol and cotton swabs or paper towel, but be careful not to go too hard on the dies.

Don`t use too much thermal paste, just enough to cover some portion of the die. If you have a spreader, use it. The thermal paste will apply evenly after you tighten the heatsink.

Replace the thermal pads if necessary. Be sure to use the same thickness.