First things first. Look for a brand name for the door. Look on the door handle, in the corners of the glass there may be an etched in manufactureres name, find the manufacturer.
Now go to the local lumber yard/home center and order some new rollers for the door bottom.
Now you are ready. Get a pair of sturdy saw horses set up outside by the door. Lay some old towels or padding of some kind on the saw horses.
Now slide the door open about 8" and place a small flat pry bar in the middle of the door under the bottom edge. Grab the sides of the door firmly and gently take your foot and press down on the pry bar which should raise the door up. As the door travels upward, gently pull out on the lower portion of the door, allow the door to drop down out of the top track a little which should let the entire door panel free of the frame.
Lay the door on the saw horses and go about replacing the bottom rollers. Also clean the bottom track of the threshold to get dirt grime out of the grooves and channels in the door bottom.
Re-install the door by slipping the top of door up and into the top frame
Use the same flat bar and place the door bottom on the flat bar to hold it in place. Now lay down so you can see the bottom rollers of the door. Take a small flat screwdriver and push up the rollers so they are above channel in the bottom frame. When both rollers are in line, let them drop down onto the raised spline of the bottom track. Adjust rollers up/down so door glides smoothly.
* Get a buddy to help you for about an hour - the doors are pretty heavy.