If your Frontier or Xterra has electrical connectors and a rubber boot, disconnect the connector first and pull off the boot. Carefully slide off the holding screw, remove the light bulb from the light fixture, and keep your fingers off the glass surface. Slide the new bulb fitting into the hole in the headlight, and lock it in place with the retaining ring or clip. After putting everything back together, attach the electrical connector, put the rubber boot on (if you have one), check the headlight, and close the hood.