I only know two ways to shorten belts with tapered ends.
Assuming it has cartridge loops, you would have to cut a new taper at one or both ends and possibly remove a few of the loops. Re-stitch the edges if there is a stitch line. This method works with both lined and unlined belts.
The other way to do a lined belt is really not worth the effort in my opinion but it could be if someone has a sentimental attachment to the belt and is willing to pay for your effort. With a lined belt you could cut the belt at the center of the back and remove an inch or two from the outside of one end and the liner on the other end. Skive the ends so when you re-assemble it, you can glue those back together. That done, re-stitch the section top and bottom to prevent it from separating. If you are careful about where you make your first cut, the overlap seam will be right at the center of the back. If there are bullet loops all the way around, you would have to unstitch those first down one end and put them back after splicing the belt back together. In that case, make sure the amount you shorten it is equal to the length of 5 or 6 bullet loops. Also, if the loops are stitched between the layers, you would have to separate the two layers on the end where you remove the loops and stitch them back before putting the liner back on. If the two layers are glued together, you would have to separate them with solvent first. This method can be more work than making a whole new belt.
I hope this helps.