Removing the Old Door
Replacing a screen in the door or the entire door requires taking the door off the track and out of the door assembly. These doors are normally made of aluminum and they can bend and break rather easily, so care must be taken when removing them. There are two tension screws on the bottom sides of the door. They will need to be loosened to lift up the wheels that move the door along the track. Once the screws are loosened a flat head screwdriver can be inserted under the door to lift the wheels up even more. The bottom edge of the door can be pulled right over the track. Then the door can just be moved away and pulled out of the top track.
Finding the Correct Door
Not all sliding screen doors are the same. Purchase a screen door made by the same manufacturer as the entire assembly. Try to match the old door perfectly, if you can find a model number or still have the original installation instructions, take them with you to the store. The problem is that all manufacturers make different latches and the door will not close properly if it doesn't match the entire door assembly. If you can replace the old door with the same exact model than purchase a non latching screen door. It will simply pull over to the frame, but you will not be able to latch it or lock it shut.
Installing the New Door
When putting in the new door, make sure you have the handles toward the outside of the opening. Lay the door down on the floor and make sure the tension screws in the bottom are loose. This will allow you to get the door over the track. Tilt the door so you can put the top in first and then lift it as high as you can inside the track and try to put the bottom in. If you can't get the bottom in this way, use a flat head screwdriver on the front wheel, pushing it up. Put the front in and then move to the back wheel. Once you have the door in the track, tighten the tension screws to lower the wheels thus keeping the door from jumping out of the track. Some doors have screws at the top corners that are used to level the door. If your door won't slide back and forth easily, adjust one or both of the screws at the top.