You need to earth them up to get more crops. Plant will produce more that way. As others mentioned, do not let them get exposed to sun, they'll turn green and you shouldn't use those.
Other than that, if they are big enough and healthy looking, did'em up and eat them. You don't have to harvest all of them right now, you can just take what you need untill greenery begins to die down. Then (autumn time) harvest the lot, let them dry in sun for a bit before you store them somewhere dark and cool.