Disconnect the ribbon cable by gently using a plastic opening tool to flip up the black connector latch. The cable should easily slide out from the connector.
The motherboard should now be free from the rest of the device.
Remove the plastic buttons and keep them in a safe place.
All of the buttons have "key" notches to prevent them from being inserted incorrectly (backwards or in the wrong place), but it can take a while to figure out where everything goes if they get shuffled around.