Wires are used to maintain the shape of bonsai tree. If you want to remove a wire from any branch or the trunk, use a special tool called bonsai wire cutter for this purpose.
This cutter will help you unwind your wire smoothly without damaging the tree. You dont want to leave ugly scars on your plant by not removing the wire properly!
These wire cutters do a great job with helping you avoid that. They are cheap and effective, so get yourself a pair of these wire cutters.