how to replace 3v bulb of torch with LED?
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.
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Have you looked for a direct replacement? Without knowing which bulb it uses I can't tell you if there is a direct replacement. Ace hardware and Radio Shack both now stock led replacement bulbs for flashlights. But since you used the word "torch" I'm thinking that you might not be in the US and don't have any RS's near you.