Depends on what is going on BEFORE she mounts her.
Dogs will mount in an excited mind state. And does NOT always mean dominance (scientifically proven).
DO NOT use corrections, this does nothing unless your correction is harsh enough to leave a lasting imprint in her brain - but this could also have a negative impression on the pup (which you don't want).
Keep track of your older dog's body language right before it happens, what is going on? Is the puppy doing nothing? Is the puppy running around?
You need to catch her JUST before she goes to mount the pup and use a positive interrupter (such as a low whistle, kissy noise). Please condition your dog to a positive interrupter
http://www.youtube.com/watch?v=TBvPaqMZyo8 .