Remove the screws that hold the cooling assembly and disconnect the fan.

Now is a good idea to upgrade the RAM.

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.

You can also disassemble the fan and clean it from the inside.

It is also recommended to use compressed air to remove any dust in the laptop and the fan.

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 to ensure even pressure. Dont forget to plug in the fan cable.