Unscrew the 4 screws holding the CPU heatsink and lift it up, gently.

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

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

Last, put the heatsink back carefully and tighten the screws in order 1 -> 4 to ensure even pressure.