I am not sure I am quite understanding exactly what you hope to achieve. Why do you want to change this? Labs, in general, are not overly assertive dogs. They have a tendency to have happy-go-lucky lucky personalities with a "can't we all just get along", goofy demeanor. Most think every dog and person is a long lost best friend. My 3 y/o male is even fairly submissive with other dogs in some situations.
If you are looking to boost her confidence my first suggestion would be to get in to a nosework class. I have done over a year of this with my own boy and have watched it work really well to help reactive, shy dogs come out of their shell.