Turn the motherboard over and disconnect the fan.

Remove 4 small screws which retain the fan.

Turn the motherboard the right way up again and lift off the fan. remove any build-up of dust, and blow through the radiator fins to ensure they are clear.

It is best not to remove the heat sink and heat pipe from the processor if not strictly necessary since new thermal paste, carefully and correctly applied is required when refitted, otherwise the processor is liable to overheat, causing it to spontaneously shut down.