With that behavior try this: tether the end of the leash over a doorknob, under a table leg or over a fence. She should be completely comfortable and able to sit or lie down. The leash is not tight unless she moves to the end of it.
Start moving towards her and say, "Yes" or 'click' and toss her a treat from a few feet away. Then, take another step closer and repeat the exercise. Do this until you are right next to her. If she starts to mouth you, say, "Leave it" and quickly move away from her until she calms down. Then, continue the exercise by again moving closer and rewarding BEFORE she bites you.
If each time she bites you "the fun ends" and you walk away, eventually she will be less motivated to do this behavior. The key is to reward before she bites you and also to have well-timed "Leave its" if she goes towards you.
On walks you can simply put the end of the leash over a fence and walk away each time she bites. But, pay attention to situations where she normally does this behavior and reward before she does this. Fast movements often entice a puppy to bite, so try and walk slower until she gets back on track.
It might prove useful to bring her favorite toy on walks and play tug with her or allow her to hold it. Puppies often just like something in their mouths.
Ask if you have further questions.