I'm not an expert with vent prolapse and you might be able to find more thorough advice by searching old threads on here, but you should gently clean with tissue with warm water and spray with veterycin or something similar. Sometimes it works to gently push the tissue back inside, but it;s important to keep any tissue that remains prolapsed moist with vaseline or ointment. When the inflammation goes down the tissue may pop back in on its own. It's also very important to keep her isolated as other birds will peck at the exposed tissue.
I hope your girl makes a full recovery, but prolapses can be difficult/serious. Also once a hen has a prolapse the tissue gets stretched out and it may just keep recurring every time she has lays an egg or even a large poop so you'll want to keep a close eye on her for awhile.