I have never had to replace the fabric on one, but if I did, first I would find out the manafacture, then messure it and call camping world.
My brother used to work in an RV shop. He was telling our Dad how to do it since his MH needs one replaced. I have installed the slide awnings new, but never a replacment.
I have a feeling you will need 3 people to do the job the first time. One on each side pulling the new fabric into the channels, and one on the side keeping the fabric straight as it feeds into the channles.
As far as needing to unwind one turn, just unwind it, then lock the roller.