Leave the leather piece in the sun. If you have the time, this is the most natural way to lighten the leather, and it adds no harmful chemicals. Place the leather outside on sunny days and monitor its progress. Once it has lightened enough, keep it out of the sun. This process is slow and can take several weeks or months, depending on how light you want the leather.
Mix leather bleach with water, if you don't want to wait for the sun to do the bleaching. Leather bleach is safe for the material, unlike chlorine bleach. Follow the instructions on the container. Start with a diluted mixture of 1 oz. bleach to 2 cups of water. If this doesn't lighten the leather enough for you, then you can make a more concentrated mixture and apply a second application.
Brush on the leather bleach mixture with a small, soft brush.
Leave the bleach on until it dries, and then wipe with a lightly damp rag. Too much water might darken the leather. You only want to remove the residue.