Who is the manufacturer of your awning?
In general, if you have a "modern" awning like A&E, Carefree, or Zip-Dee, you would first want to acquire the replacement awning (fabric), then you can extend your old awning out until you can see the slot where the fabric slides onto the roll. From here, you lock the roll, and support the assembly, if it isn't self supporting, and you remove the awning fabric by sliding it out of the tube, and out of the awning rail on the trailer. Some screws may be in place to keep it from sliding out unintentionally. Reinstallation is the reverse.
Some manufacturers require you to remove the awning tube from the arms, and at least one of the tube endcaps has to be released to remove the fabric. This is the place where things can get dangerous, as you need to make sure you are not working with a spring that is wound up with energy waiting to release on you.