The awning strap is attached to a piece of plastic about 1/4" diameter x 2" long which slides into one of the grooves in the awning drum. You should be able to get a replacement strap at any RV dealership.
You should be able to unwind the awning without the strap. Have a person at each end of the awning (you'll both need step ladders), flip the unwind latch and carefully unwind the awning from each end. If you stop unwinding to re-position the ladders, the awning tube won't roll back up as it is latched to unwind. Once it's fully out, slide a new awning strap in.
Edit: I'm a little slow at typing so I'm adding to what Glen said.
Nights camped in 2013 - 55, 2014 - 105, 2015 - 63