The pictures help greatly.

The fringe you have is the simplest form, as the edge of the front apron has been folded back, and then folded forward again, zig-zag fashion, and stitched all the way down the edge of the apron. A half-inch or so has been left standing proud of the stitching, and this bit has been frayed to form the fringe.

I think that, to get with effect you want, you will have to undo the stitching along the base of the fringe, down the edge of the apron, in order to access the 'pocket' in which to insert the new piece of material for fringing.

You will probably have to undo the stitching along the inner edge of the fold-back, so as to provide enough of the original fringe to be visible behind the new fringe when that is in place. I have to say, I think a contrast fringe will look good on that kilt.

You could also try removing the waist-band piece, too, and replace that with the new contrast cloth, if you think the kilt will take it.

As for pictures, Google & Co. usually answer well enough, but Barbara Tewsbury has already written the last word on kilt-making 'how-to' (her contact details are on the right hand side panel) and the instructions in her book are easy to follow.