For original Steam Decks: Use a Phillips driver to loosen and remove the two screws securing the heatsink to the motherboard:

One captive 3.5 mm screw

One 3.4 mm screw

No, your eyes don't deceive you! The third heatsink screw was removed earlier in the disassembly: it doubles as the hidden board shield screw.

For refreshed Steam Decks: Use a Phillips driver to remove the three screws securing the heatsink to the motherboard:

Two 2.9 mm screws

One 3.7 mm screw

During reassembly on both versions, tighten these screws in order (1, 2, 3) as stamped into the heatsink.