Check the type of fabric the awning is made of--like canvas or vinyl--along with the size and dimensions of the fabric. This will make sure you get the correct size and type of replacement fabric.
Remove the old fabric off the awning frame. If you can cut it off the frame, it will be easier. Don't cut it off at the mounting channel; pull and slide it out of the channel. Untie and remove the ropes and whatever pieces of fabric remain; take note of how the rope is tied and routed through the frame.
Slide the new fabric into the mounting channel, which should be at the top of the awning. Drape the fabric across the frame, and tie it in place the same way the old awning was tied; use new rope if the old rope is dirty and worn. Read over any instructions that came with the new awning fabric and the old awning frame if you have them.
Unwind the awning fabric from the awning rail. Release the travel locks and unravel the awing by about a foot. Lock the tube's spring tension with a cotter pin inserted through the holes in the end caps. Disconnect the arms from the side of the vehicle, and use them to walk the fabric off the rail.
Disconnect the arms from the awning; you usually need to insert a vise grip up through each arm and clamp down on the attachment plate inside each arm; then remove the bolts holding the arms to those plates.
Remove the cotter pin from the awning tube, and unwind the tension on the spring, counting the number of times you unwind until all tension is removed. There can be a lot of tension in the spring, so use extreme caution. Remove the torsion assembly by drilling out the rivets holding it in place, marking the cam handle's position so you know where to replace it.
Unroll the old fabric off the tube, and slip it off the tube. It may take two people to remove the old fabric and slip the new fabric onto the tube. With the fabric centered on the tube, tuck in the cords at each end of the fabric.
Attach the torsion assembly back on the tube with new rivets. Place the vise grips back on the same end pieces. Carefully wind the spring back to the correct tension and have your assistant re-insert the cotter pin. Attach the arms back onto the awning.
Connect the whole assembly back onto the rail along the wall by feeding the awning cord into the rail; this may require three people as the other two hold the arms. Remove the cotter pin, roll up the awning fabric and connect the arms back to the wall.