Yes this is possible but a little difficult. The biggest problem is getting the color as dark as you want and the new stain will 'slide' down the wall until it has dried. To minimize this and get a dark color I recommend using a gel stain.
Preparation is very important. Wash all surfaces with water and a mild soap, rinse well and let dry. Now lightly sand, 150-180 grit paper or fine sanding sponge. Wipe off the dust and stain.
Once the new stain is applied and dry you will need to seal it with a varnish or other clear protective wood finish.