Remove the battery.

Remove the 2 screws holding the CPU panel.

Dont forget to disconnect the fan cable.

Remove all screws that hold the cooling assembly and carefully wiggle it out of the laptop.

Now is a good idea to upgrade the CPU if needed.

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.

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.