Unlatch the hood of your Civic and prop it against its support brace. Locate the faulty headlight's assembly against the inside of the engine compartment.
Disconnect the wiring harness from the headlight bulb, pushing the sides of the connector together with your fingers, and pull the connector off of the headlight bulb.
Grasp the plastic bulb cover with your hand and twist it counter-clockwise until the cover can be pulled off of the headlight assembly. Pull the rubber dust cover off of the rear of the headlight bulb.
Pull the damaged headlight bulb out of the headlight assembly and discard it. Insert a new bulb into the headlight assembly, taking care not to touch the glass portion of the bulb.
Place the rubber dust cover over the rear of the bulb and press it into place. The bulb's prongs should protrude from the center of the dust cover. Place the plastic bulb cover over the headlight bulb, twisting it clockwise to lock the cover onto the headlight assembly. Press the wiring connector onto the bulb's prongs until it is seated securely on the bulb.
Turn on the car's headlights and check that the bulb is working properly. Cycle between high and low beam. Check that the wiring harness is connected properly if the bulb is not working.