E.A.
I would tell you not to do anything. Self soothing behaviour is usually not harmful, and trying to stop a child from doing something in their sleep in pretty much impossible. There has even been a big turn around in recent years regarding thumb sucking. Most pediatricians will tell you to just let it be. Why do you want her to stop? Maybe examining why you feel the way you do about it will help you come to some kind of acceptance for now, most kids grow out of it before age 5.