Step 3: Dismantle fret button assembly
Use a large-ish flat screwdriver to remove the circuit board. It is quite a tight fit on the plastic posts, but if you're careful you shouldn't damage the board.
Carefully lift the circuit board and set it aside. Be particularly careful not to trap or fold the ribbon cable at any point as it's quite delicate.
Remove the blue rubber pad from the buttons, and set it aside - this will reveal the fret buttons. You can now remove these too.