If it's a manual awning and the center pull strap is missing or got rolled up into the awning, then you'll need two people to get it down. Using your awning rod (you do have one??), release the lock tab at the front end of the roller. Usually, from the rolled position it just pulls down. Then with two brawny lads, pull the arms away from the side of the trailer. Best to pull from near the roller. Once it's down, it should relock itself. If the strap is missing, a replacement can be purchased at any RV place for about $6.
When rolling the awning back up, you will definitely need the strap and the rod. When you release the lock, make sure you have a good grip on it or the awning will roll up quickly and everyone in the campground will know it when it slaps the side of the trailer. DAMHIKT.