Too wide - that is the easier one to adjust. Assuming that the axle is the correct length and only the wheels aren't fully seated, put the wheelset into a vise and gently close the jaws to squeeze the wheels inwards on the axle. If the axle is also too long then you need to make a spacer with a hole for the axle. Then put that spacer between the wheel face and the vise jaw, then squeeze the wheel with the vise to move it inward.
Make sure that the axle is perpendicular to vise jaw's faces and don't try to use pliers or some other tool with non-parallel jaws. I suppose you could also try to hammer the wheels inwards, but I would not be keen on doing that.
