Remove the two Phillips #00 screws.

Next, remove the plastic mount for scroll wheel.

The top button board can now be removed.

Do not force it if it doesn't lift up easily. Go back to the header pin desoldering step and check for any still remaining connections.

If you have modded your mouse to use socket instead of soldered connections, lift the board up from the locations indicated by the yellow circles to avoid breaking the PCB. When reassembling, first align the two center posts that the scroll wheel mount attaches to the holes, then press down on the header area.