Use a 3 mm hexagonal wrench to release the heatsink. The wrench must be at least 15 cm long.
Release the holding bolts ONE TURN at a time, following an X pattern (top left, bottom right, top right, bottom left) and continue until you hear a release click in each of the bolts. Note: do not completely undo one bolt and then move to the next. Repeat the X pattern, making one turn at a time, until the heatsink is released from the PCB.
When you replace the heatsink use the same pattern to tighten the bolts. Do not tighten more than finger tight. Make sure that the same tension is applied to each of the bolts. Uneven pressure can result in the CPU contacts failing. Symptoms range from boot-failure to missing or malfunctioning memory cards.