It depends on the model of flashlight/torch that you have. If it's a maglite or maglite-style, there are drop-in conversion kits available wherever maglites are sold. In those cases, it's as simple as changing a bulb. If you have a different style of flashlight, of course this isn't going to help you. You'll need build or buy a control circuit and an LED, and then figure out how to fit it in there. There are plenty of LED instructables that should get you started, and dealextreme.com is where I suggest you might look for parts.