Remove the three 7mm screws surrounding the metal fuel filler mouth. Pull the loosened fuel filler neck away from the body.
Use penetrating oil, like WD-40, and a 5/16th socket to loosen and remove the metal bracket holding the mouthpiece to the filler neck.
Pull the tube and the metal piece apart using the flathead screwdriver. This will break the adhesive bond and allow you to pull the filler neck free from the tube.
Slide the new filler neck into the filler tube. Make sure you use penetrating oil to lubricate the two pieces. Tighten the clamp using a screwdriver and 5/16th socket.
Reattach the filler neck to the truck’s body by fastening the three 7mm screws at the mouth of the fuel hole.