Sure, the odds are slim that you'd ever be faced with an atomic device ticking down to zero. But think of how Jack Bauer it'd be if you were. And then who're you going to trust? Us or some do-gooder rock band?
Disconnect the wires leading from the battery or the trigger/timer to the detonator. The battery will look like a battery; the timer is the part that's counting down. Be careful: This is the trickiest part of the bomb – and where it's most likely to be booby-trapped (the whole red wire/blue wire thing comes into play here). And for God's sake, don't fuss over the process in order to make the timer stop at something clever like 0-0-7.
Remove the neutron trigger. This will be a small disc or ball. Don't eat it – it will be made of polonium or some other highly radioactive material, which initiates the chain reaction. After this step, the bomb can still detonate, but it won't be Hiroshima.
Remove the conventional explosive. It's the first part to go boom. In government weapons, it'll likely be some type of IHE (insensitive high explosive), which is fairly safe to handle. But in an improvised device, it might be considerably more unstable. Just keep a steady hand…
Separate the U-235 masses. These will be two small but very heavy chunks of metal – an isotope of uranium. If they get too close to one another, the combined critical mass will flood the area with radiation, and you will die. Kept apart, each emits only relatively harmless alpha particles – you could even handle them without gloves if necessary. Put each piece in a separate metal box and call the authorities.
If these tips don't work, give us a call and let us know what we got wrong.