In order to replace the rollers, you need to take out the door first so that you have a clear field to work. Most doors have two screws, one at each side that hold them in place. Use a screw driver to turn these screws in an anti clock-wise direction to loosen them up. As you loosen up these, the door sinks lower and breaks free of the support. Ask some one to help you get it out of the track. Lift the bottom part first, and then take off the top part.
Locate the roller assembly at the end of the door. Loosen up its screws to take off the roller. Use the screwdriver to turn the screws in an anti clockwise direction so that the rollers come out free.
Use these rollers as reference when you go to buy the new ones from the hardware store.
Now that the rollers are installed, it is time to put back the door in its original position. This time , slide in the top first and than lower the bottom part in place. Tighten the door screws at both ends.
Check the door for snug fit and smooth slide.