Unscrew the 4 screws holding the CPU cooler. Disconnect the fan cable by releasing the latch on the connector and pulling the cable out.

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

Once you removed the old thermal paste its time to apply new one.

Dont 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. Dont forget to plug in the fan cable.