By “reduce,” do you mean you want to let it out? Removing shoulder pads is simple. If they’re sewn inside the garmet, find the inner seam that connects the lining to the jacket itself, remove only the number of stitches necessary, take the pad out and whip stitch (or choose a stitch, I suppose) and sew the area back together. They may also be sewn through, so you may have to remove some of those stitches too. The easiest way to fix something to big in the shoulders, assuming it has a seam down the middle of the back, is to take that in. You could also undo the seam connecting the shoulder fabric to the sleeve and cut some of that down, or sew it into itself, then reattach the sleeve. Where are the seams in this jacket, what is it made of, and what exactly do you want to do? A little more description and I should be able to give some better advice.