Had this problem too.
Start by putting the assembly on a flat surface with the curved side facing upwards.
Remove one of the end caps by gripping it under the slot that the material passes through and moving it upwards and off the end of the assembly.
Put this part to one side.
Then remove the tiny cross headed screw that becomes visible.
Then turn over the unit and start to remove the screw from the opposite side. The part that these two screws hold in under tension so be careful to stop the part rotating until you want it to!
With both screws removed you can prise the end plate slightly out of the end of the channel and allow it to rotate (I used a finger to stop it every half turn to count the tension required when reassembling - 26 half turns in my case).
Now that thension was released I removed the end cap from the other end of the assembly in the same maner as above but not worrying about tension on the end plate.
With both ends free from the main assembly i pulled out the tensioners from either end (these are what the end plates are attached to inside the main channel)
I then removed the inner tube roller to which the fabric cover is attached (partly attached, as I wouldn't be doing this otherwise!)
I reattached the fabric with a 6" x 1" piece of double sided tape, then rolled the fabric onto the inner tube roller, and slid the tube back into the channel.
Then I attached one endplate/tensioner and once secured, went to the other end and wound the tensioner 26 half turns (as above) then pushing the end plate into the main channel I secured it with the remaining two screws.
After pushing on the end caps, I fitted it back to the car - brilliant - sorted at last! When retracted there is some creasing to the fabric which has been like this since the fault first appeared - I hope it will return to normal over time! Good luck!
Murf-pug206, November 2012