Grafting a torn bit of tissue back onto the wound only works if you do it immediately following the injury. Notice the blackening of the comb where it's detached. That means there is no blood supply there. When blood supply has been cut off, the tissue quickly dies. That blackened tissue is dead.
Now, you may either leave the flap alone and it will eventually fall off on its own, or as
@raingarden pointed out, you may trim the loose flap off using sharp scissors, making sure you cut away all the blackened tissue.
It will bleed when you do this, so have some flour or sugar or corn starch handy to put on the bleeder. It will cause a bit of pain when the comb is dubbed, but chickens deal with pain well, and it will only hurt for a few minutes.
To ease the chicken's reaction to the surgery, give her some Gatoraid to drink or mix some sugar into her drinking water for the rest of the day.