We pull the door down section by section on the affected side, working the roller stems free of the hinge brackets without dropping the section out of the track. Each roller gets checked against its neighbors - if one's cracked, the two or three around it are usually close behind, so we replace them as a set rather than one at a time. On a double door with 16 or more rollers per side, that's most of the run. We reset the track alignment while the door's open, since a bent or slightly splayed track is often what accelerated the wear in the first place, and worn rollers get replaced with sealed-bearing nylon, matched to the stem length your hardware already uses. We test the full cycle before we call it done - quiet, square, no drag at the bend where the vertical track curves into the horizontal.