Nail biting is a bad habit. First you need to recognize at what times you tend to bite them. Is it out of boredom or stress? If it is, you need to find a different outlet. If your bored, find something fun you can do with your hands or fingers. If its out of stress, again have something that occupies your hands like a stress ball, a stone to play with, coin, etc. something smaller that you can carry with you. Also you might try sunflower seeds for the hand to mouth association.
Good luck