Place a sheet in the bottom of the tub or shower stall to protect the finish. Locate the screws that secure the lower guide to the front of the bottom track. Remove the screws with a Phillips-head screwdriver, and pull the lower guide away from the track.
Slide the inner door panel halfway open, and grasp each side of the panel with your hands. Lift up the panel while swinging the lower part of the panel toward you. Carefully pull the panel down and away from the upper track. Place the panel on a flat surface. Repeat the process for the remaining door panel.
Find the screws that secure the rollers to the bottom of the shower door panels. Remove the securing screws with the Phillips-head screwdriver and remove the rollers from the doors.
Take one of the rollers to a home improvement center to purchase replacements. Ensure that you purchase rollers that match the diameter of the current rollers exactly.
Insert the stem of the new roller into the slot on the door panel. Hold the roller in the middle of the slot, and thread the securing screw into the roller stem from the other side of the panel. Tighten the screw with the Phillips-head screwdriver. Repeat the process for all the rollers.
Lift the outer panel by the sides and insert it into the upper track. Position the lower part of the panel into the lower track. Slide the door back and forth to ensure the panel glides smoothly. If necessary, remove the panel and loosen the securing screw on the rollers. Slide the rollers up or down within the slot as necessary and tighten the screw. Install the panel again and check the operation.
Repeat the panel installation for the inner panel just as you did for the outer panel. Adjust the rollers as necessary until the panels slide smoothly inside the shower door tracks. Place the lower guide over the front of the lower track, and secure with the retaining screws.