One side at a time, insert a pick next to the nosepiece, slide it to the edge and pry.

When both sides are loose, carefully lift the cover out, and fold it so that the nose piece is on the outer perimeter of the device, as shown in the second image.

The proximity sensor is still connected via a flex cable, so be careful to not rip or crease the cable.

Lift the small black clip on the ZIF connecter up, and pull the flex cable out.

The cable is lightly adhered to the headset so be careful when removing it.

During reassembly, make sure to position the cable correctly. Latching the small black clip back on shouldn't require much force. If you don't connect the cable correctly, the headset's screen may stay black and the device will turn off after a few seconds.