Swapping a single nylon roller on a door that's unloaded - meaning the spring is holding the weight, not your hands - is within reach for someone comfortable with a socket wrench and a ladder. You pull the roller bracket, slide the old roller out of the track, drop the new one in, and reset the bracket. Where it gets dangerous is any roller near the bottom bracket on a door with a wound torsion spring, or any job where you're tempted to remove the bottom bracket itself - that bracket carries cable tension and has put people in the hospital. If more than two or three rollers on the same side are worn, that's usually not a roller problem anymore; it's a sign the track itself has drifted out of alignment, and chasing that without the right gauges just wears out the next set of rollers faster.