Your first step is to remove the screen door from its tracks by turning the adjustment screws counter-clockwise to release the tension on the wheels and lowering the door. Inspect the tracks, top and bottom. If they are bent, straighten them with pliers or hold a block of wood against the bent parts and rap the block with a hammer. Some doors have wheels at the top as well as the bottom. The top wheels are not as likely to be worn, but if they are, replace them. If the corner assembly screws are accessible, use a screwdriver to make sure they are tight on the sliding screen door installation.