Again using the screw driver, remove the three screw located on the side of the exterior metal case.

Next by hand or with pliers, rotate both handles/levers protruding from the metal case anti-clock wise to remove.

you should be able to remove the case now by sliding it up, be careful when doing so as not to loose internal parts now loose.

if the smaller lever is too stubborn to rotate, you should still be able to bend the case over it to remove the metal exterior.