how to relace a headlight on a 2002 trailblazer?

Answer 1

The front grill has to be removed to change a headlamp bulb.Something better done at a repair shop or the dealer.

Answer 2

First thing to do is to buy a service manual.

Failing that, the halogen capsules usually go in from the back of the headlight assembly. You shouold be able to see them from the engine compartment.

If you are talking about replacing the actual headlight assembly, they are usually also attached from the rear. Probable with 3 or 4 10mm nuts.