Once you open the back panel, it should look like this. Now is a good time to upgrade the HDD and RAM.

Disconnect the fan connector and unscrew it with the cooling assembly.

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.

Now is a good idea to upgrade the CPU.

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.