How to cut a stitched leather belt without it unraveling?
Without seeing the belt - but having an idea of what it might look like - here's an idea that might work:

I assume the buckle end of the belt is pulled through the lower part of the buckle and folded back onto itself and sewn across the edge to hold it in place.


To remove the buckle, carefully cut that stitching open and pull the belt through to the length you want and trim it so that the new edge hits the back of the belt at about the same place as before. 
Sew across to hold the belt in place.
The cut threads along the edge of the belt will now be on the underneath side of the belt.  If you don't fix them in place, they will unravel and eventually show on the outside of the belt. You can use Fray Check (fabric glue found in craft or fabric stores) or superglue to glue the cut threads to the underside of the belt so they won't unravel.


You can also take the belt to a shoe repair place to resew the cut edge, unless you have an awl (sharp hole-maker) to make new holes in the new belt edge that you can hand-sew through to fix the folded-over part to the back of the belt.  Then glue the cut thread as above.

If the sewn edge of your belt is a thin leather ("boondoggle") and not thread, you can try gluing the cut stitches, but you can also have a shoe repair place sew them down when they sew across the new edge.

Good luck!  Good for you for trying to fix the belt you like instead of just tossing it.