Re-staining something that's already finished requires stripping (removing) the finish. Stripping finish is best done with a chemical remover. If it's an old clear finish, or a lacquer finish, it should come off easily with something called a furniture refinisher:
http://www.minwax.com/products/specialty_products/antique_furniture_refinisher.html
Using this product requires chemical resistant gloves (usually sold where the remover is, and 00 steel wool)
This product does not work on paint, polyurethane or modern finishes, which require something called paint and varnish paste remover. If you use this product you need gloves, plenty of rags and a wide putty knife.
With both products make sure you work in a well ventilated area, preferably outdoors in the shade. Do not work in a closed environment or outside in direct sunlight.
Once you get the finish off, sand the wood with 150, then 220, going with the grain of the wood. If it's veneer be careful not to sand too much. If you're not sure, don't do too much sanding in one place. Then you stain with a black (sometimes sold as ebony) stain and apply clear finish.