If you look close at the slot at each end of the tube, you will see where it's crimped closed. Take a narrow straight blade screwdriver and gently pry the crimp open at both ends. Once open you can slide the old fabric out the end. If you look at the fabric you will see a stiff string with the fabric folded over it. This is what holds the fabric to the tube. I cut a 1/16" length of piano wire, folded the fabric over it and used contact cement to glue the fabric to itself, creating a hem with the wire in it. I then slid the "hem" back into the tube slot from one end and tacked it with super glue. You can open the slot with a wide blade screwdriver if your fabric is thick. Be gentle. This is how I did mine. Hope this helps.
Mal