If I were altering them width wise without affecting the selvedge then the only way to do that would be to undo the inside leg stitching right up to the groin and and make it narrower that way by bringing the hem inwards. The main problem you will encounter when you do this is that the leg will taper downwards and make the jeans look a bit silly because the overall width at the top of the jeans around the hips and seat will remain the same.
The cheapest way to reduce the overall width would be to simply wash them in a very hot wash then immediately dry them in the tumble dryer, the whole of the jeans will shrink in proportion and be a slimmer fit anyway, but of course the waist might be too slim then as well.