yes you should deadhead them they have already dropped their seeds if they are brown. Be sure to cut them to the nearest healthy leaf at a 45 degree angle to let any water drip off to prevent fungus. These are annual plants so they are going to need to be replaced. I'm sure some of the seeds dropped off will germinate next year but I feel it's always better to plant seeds in spring and watch them do their thing. Don't overwater and keep them in sun and they'll look great.