Remove the six Philips screws securing the heatsink assembly to the motherboard.

Carefully lift the heatsink. If it won't come off, twist it as you pull it off. If it still won't come off, pull harder.

Unfortunately, if you have to resort to pulling harder, the processor will likely come off with the heatsink. If this happens, use an adjustable heat gun set to ~200C to soften the thermal paste, first.