You might not need to replace it.. Often there is enough fabric on the roll you can simply reverse it
Extend the slide. Pin the roller (You will need to find instructions for that) and bring slide back in a bit, You can then dismount the roller. On the body end you may have to remove a screw or two and "Spread" the channel a bit. then slide the thing out or pull the rib and pull it out (id' slide rib and all. or to be more precise I did) unroll fabric. but body end in roller. put roller end in body and reassemble. If there is enough fabric, if not ... take old fabric to RV store or other (Canvas tarp) store and they can provide replacement.