Disconnect the upper fan connector from the motherboard.
Remove the four spring loaded screws holding the heat sink assembly to the motherboard.
When reassembling, ensure that all the screws are equally tightened: thread all screws one turn, then add one turn to each screw in the following order: top left, bottom right, bottom left, top right. Repeat this order until the assembly is firmly seated.
Slide the heat sink assembly including the upper fan out by pulling it down and left simultaneously.
When reinstalling the heat sink, ensure it is thoroughly cleaned and the thermal paste (or pads) are replaced to ensure proper thermal contact with the devices on the logic board.