That dark walnut stain may have been a good idea 10 years ago, but styles change. If you want to remove the stain to match a lighter decor scheme, you need to remove the finish first, but there is more than one way to do that. Sanding the wood usually removes most of the stain, but bleaching may also work, and it is a safer option for thin veneers.
Removing the Finish
If the stained wood has a finish, it's either shellac, lacquer or some kind of varnish. Often, the easiest way to remove a thin lacquer or shellac finish is to wipe it off with lacquer thinner -- for lacquer -- or denatured alcohol for shellac. You may also be able to sand it off without much trouble. Cured finishes, such as polyurethane or alkyd varnish, won't soften with a solvent, so you have to strip them with a chemical stripper. Apply the stripper with a paintbrush; wait for the finish to bubble up, and then scrape it with a paint scraper. After neutralizing the stripper with water, you're ready to work on the stain.
Sanding Out the Stain
Sanding the wood with a progression of increasingly finer sandpaper grits removes most of the color left by an old stain, and if you're applying a new stain, it should mask any residual color. You may have to remove as much as 1/16 inch of wood or more to get all the color out, and that's much easier to do with an orbital or belt sander than by hand. After you've finished machine sanding, always sand by hand with the grain of the wood -- using the same grit sandpaper you were using in the machine -- to erase machine marks.
Mixing a Chlorine Wood Bleach
Dye stains sink deeper into the wood grain than pigment stains and can't always be removed by sanding. Chlorine bleach can lighten most dyes. Make a strong chlorine wood bleach by using calcium hypochlorite, which is used for chlorinating swimming pools. Mix a saturated solution by adding crystals to water until no more of the crystals dissolve. Chlorine bleach isn't effective for removing pigment stains, but you may have some success if you bleach these with two-part wood bleach; the two parts are hydrogen peroxide and sodium hydroxide. Two-part bleach comes as a kit that usually includes instructions for using it.
To bleach out a dye stain from a piece of wood, apply the saturated chlorine bleach solution immediately after mixing it; the bleach quickly loses its effectiveness if you leave it standing. Wearing rubber gloves and a respirator, and brush or wipe the bleach onto the bare wood. The color may immediately lighten, but you should leave the bleach on overnight to get the full effects. The procedure for applying two-part wood bleach depends on the product you're using, but you typically apply one component -- labeled part A -- and immediately follow it with part B. Two-part bleach lightens the natural wood color and must be used with caution -- it's best to test a small area first.