Stand inside the garage, and raise the level of the garage door about 2 feet from the garage floor to access the bottom roller pins.
Use pliers to grasp the metal track wall that faces the inside of the garage and runs along the garage door frame where the roller currently rests. Bend the track wall outward slightly with the pliers, just enough to be able to pull the roller out of the track.
Pull the corner of the door toward you until the roller is free of the metal track. Slide the roller with pin out of the bracket holding it to the rear of the garage door.
Slide the new roller pin into the bracket on the rear of the door, and press the door back into place, using the deformed track where you made your bend to press the roller back into the track. Lower the door, and use the pliers to bend the track wall back into its original position. Use the same process to replace the pin on the other side of the garage door.
Remove the brackets holding the pins located throughout the rest of the garage door levels in place with a socket wrench. Find the socket that fits the bolts by testing likely candidates over the bolt in the door. Attach the socket to the wrench, and use the wrench to remove the bolts holding the bracket in place. Slide the bolts from the roller pin.
Use the pliers to pull back the wall of the track where the pin rests, and pull the pin from the track.
Insert the replacement pin, and bend the track back to its original shape. Slide the bracket over the new pin and replace the bolts using the socket wrench.