At this point you should be able to compeletly remove the back cover from the main chromebook.

If some seems stuck look for a wire still held down by foil tape. and gently remove the foil tape and release the wire.

Attach the NEW back cover and connect the wifi wires. Be careful to route this sire around the hinges so that your hinge covers fit back on. You can adjust this later a little if you are unsure.