Cut the outer leaves off if they're that bad; otherwise, they'll attract pests that eat dead plant matter. Throw the leaves into your compost; they're not diseased, just "past their prime."
The smaller inside leaves will grow, and new leaves will emerge from the center.
Don't pull up the plants unless they start to go to seed ("bolt"). Even then, if you plan to save the seeds, let them flower and produce seed pods; you'll be pleased (probably
) at the number of bees, dragon flies, and other pollinators who visit flowering veggie plants.