How to change a headlight on a 2003 nissan altima?

if you mean the low beam bulb, on the driver's side take out the air box . on the passenger take out the washer fluid filler neck by turning it left 1/4 turn. for either side remove the rubber cover, press the metal hold down spring and slide it towards the bulb. pull the bulb out of the housing and pull the connector off the bulb, not the ground ring. then reverse. don't touch the new bulb.