The best way would be to trade it for what you want. The second best way would be to but leather dye and follow the directions that come with it.
Im not trying to be sarcastic and I hope I'm not coming off that way, it's just that there are several different manufacturers of leather dye and their processes vary slightly so using their directions is a must. Also I don't believe I've ever seen one that was meant for changing a color, rather they were all for dying natural leather from scratch.
Logically it should work though as you're going to a darker color.