Replacing a Carriage On a Garage Door?

"The loud grinding noise is probably the carriage wearing. The carriage is the small, black plastic piece that travels along the screw of your door opener. On the underside of the carriage is a metal rack that engages to the screw of your opener. The metal rack is made of a softer metal than the screw and will wear out over time. That part - 20414R - is an easy enough part to replace" (http://www.imdoors.com/2askkevin1.htm#MY%20GARAGE%20DOOR%20STARTS%20TO)

The question is how to replace the "carriage", and how many of them shall I replace? I think I am able to identify the portion of door that is making the noise.

hq4000