First take her to the dentist. Thumb sucking, if they are vigourous suckers who do it frequently, can change the shape of the mouth. Let the dentist guide you on when to stop.
Once you get to the point of stopping give her an incentive, some type of reward that she will want to work for. If she doesn't want to quit she won't, it's that simple. Once you get her on board, and that might take time and creativity, I highly recommend a product called handaid; http://www.yourhandaid.com/
My dd sucked her thumb from infancy until the summer before kindergarten, however she only sucked it when she was tired. She finally agreed she wanted to quit before she started k, dentist helped a bunch with this, but she didn't know how to stop. I did a bunch of on-line research and settled on the handaid. It's not cheap, but she wore it 24/7 (except to swim & bathe) for 6 weeks and when we took it off she no longer sucked her thumb. We were expecting all sorts of drama, but it ended up being very anticlimactic - she wore the handaid and couldn't suck the thumb, end of story.