Teri, the rollers are located inside the bottom frame of the door. You must remove the door.  To do this grasp the door, left & right side and lift up you will then swing the bottom out.  There is a channel in  the bottom, this is where the rollers are located.  You should find a screw on either side of the door.  These are removed and the rollers should come out.  I have found that it is easier to do this one at a time.  After you have replaced the rollers and the door is reinstalled there is a screw that adjusts the tension on the rollers.  Hope this helps you out.
                 Jim Burton
                
 July 2006