Loosen the set screw on the top. Then you have to drift the sight out from the left side to the right when looking from the rear of the gun forward (port to starboard). You can put a piece of leather against the side of the sight and use a punch to hit it. You can use a brass punch but it will leave brass marks on the sight (that can be cleaned.) You may have to hit it hard to get it to start moving but it should get easier after it moves.
Degrease the dovetail and set screw hole before you put the MGW mount on.