If the heat sink seems to be stuck to the logic board after removing all four screws, use a spudger to carefully separate the heat sink from the faces of the CPU and GPU.
Remove the heat sink from the logic board.
When reinstalling the heat sink, be sure to apply a new layer of thermal paste. If you have never applied thermal paste before, we have a guide that makes it easy.
When reassembling your device, be sure the heat sink's rubber gasket is installed correctly. The nub on the gasket should mate with the hole cut into the logic board near the heat sink fins.
The tail of the gasket should fit into the notch in the heat sink, it should not end up underneath the heat sink tab that will rest on the logic board.
Make sure the antenna cables are inserted into their respective notches on the logic board, as highlighted in the last picture.
Step 28 there is a T5 screw at the end of the heat sink which needs to be undone before the logic board can be removed this screw is hidden under the two ariel leads