At 13 weeks, it's really too early to say for sure the bite is incorrect - bites can and do change with age so you'd need to wait until adult teeth are through and check again then. If still undershot (and sadly undershot bites are probably the least likely to come right although it does occasionally happen), then you should not breed from her - faulty bites usually are genetic in origin and will pass on to subsequent generations (which is why once you have bad bites in a line, they are v difficult to get rid of) However this won't prevent you training her up etc as presumably you did not buy this puppy just for breeding purposes.