If you don't have to take it in much, the simplest way is to move the apron buckles toward the center back of the kilt _and_ move the underapron strap toward the center of the underapron. You _don't_ move the underapron buckle. Moving the buckle that's next to the buttonhole doesn't actually change the measurements any. It's moving the underapron _strap_ that does the job.

If that won't do it, it's a pretty big job that requires removing the lining, taking off the top band, undoing both the underapron and the apron edges, taking off the canvas, undoing the deep pleat and the inverted pleat, making the underapron and apron edges smaller by exactly the same amount on both edge (to keep the center stripe centered), and re-making the kilt. When you're done, the measurement across the pleats will be bigger than it would have been had you made the kilt from scratch, and the apron and underapron smaller, but it won't be noticeable unless you're taking the kilt in by more than a couple of inches (an inch or so on each side).

Honestly, if you have to do more than that, you'd be better off selling the kilt and buying a new one that fits right. If you've lost that much weight, you might as well celebrate!

Barb