To release the battery holder, simply use your fingernail or a small standard screwdriver and insert into the slot on the holder and press away from the hole. This will release the spring action and the holder will come out. See image.
w/ Screw x2 S14156, 6-32 X 3/8, PH, Truss, S/S
—How-To Video, Battery Pack
Each, sold separately.