Use a plastic opening tool to unplug the two ribbon cables connecting the two parts of the casing.

The orange-brown ribbon cable has a tiny brown plastic latch/door that flips up to release the cable. The silver ribbon is simply pressed into place; the ends are the safest point to gently pry it off to avoid damaging its wires.

If you are only replacing the camera back, stop here and reverse the preceding steps to install the new back.