Continue lifting the motherboard assembly and rotate it until it is resting next to the rest of the device.

Use the flat end of a spudger or your fingernail to flip up the retaining flap on the LCD ribbon cable ZIF socket.

Be sure you are prying up on the hinged retaining flap, not the socket itself. Damaging the socket or cable will result in a 3DS that won't have a functional top screen.