Changing the stain color on your deck doesn't have to be a huge job. If the current stain is in fairly good condition, you should be able to stain right over it with a darker stain as long as you clean it first. Changing from a dark stain to a lighter one is a bit more challenging because the old stain needs to be stripped first. There are some very effective products on the market for stripping stain and lightening the wood. Once that's done, you have your choice of colors.
Clean the deck with a scrub brush and deck-washing detergent. Power washing is another option, but can damage the wood's surface. Rinse the deck clean. If there are areas with old stain and others where the stain has worn or been scrubbed away, you need to strip the deck.
Use clear plastic sheeting or drop cloths to protect landscaping plants adjacent to or below the deck. If this isn't practical, wet them down before applying chemicals to your deck, then rinse them off as soon as you're done.
Apply deck stain stripper to a clean, dry deck with a mop, garden sprayer or large paintbrush. Depending on the product, let it work for five to 20 minutes, then hose the deck clean. It may take more than one application. If your deck is large, do it in sections so you can rinse off the stripper at the right interval.
Apply deck brightener the following day. Like deck stripper, there are several different types, but most use oxygenated bleach to lighten the wood and remove stains from mildew or mold. Wait for the time indicated on the label and rinse it clean with a hose.
Allow the deck to dry for at least one day if applying water-based stain or two days if using oil-based stain.
Test potential stain colors on a single, out-of-the-way board on your deck to make sure it's the color you want. Since the ideal deck stain is semi-transparent, the underlying color of the wood will affect the stain color. If you're applying a darker stain over a lighter existing stain, the resulting color will probably be different than the swatch indicates.
Brush the new stain onto the clean, dry deck. Do three or four boards at a time, finishing each strip of decking without stopping to avoid lap marks from the brush.