Jesus called the crowd to him and said, “Listen and understand. What goes into a man’s mouth does not make him ‘unclean,’ but what comes out of his mouth, that is what makes him ‘unclean.’"  Matthew 15:10-11

You don’t have to do anything. You can’t ask them to stop, however, you don’t want to come across as judgmental, a common perception of Christians. Someone else’s cussing does not defile you. Hearing is very different from doing. If you approve of their cussing or reply with more cussing, that’s a different story. That defiles you. You can disapprove, hear it, but not say anything. Don’t embarrass people. If they know you are Christian and what you represent, then they’ll be convicted and stop.