You should respond with class and professionalism. Remember, other potential customers will see your reply and judge how you handle unhappy customers. Never be in an emotional state when replying to a bad yelp review. If their review is angry and very reactive, thank them for their feedback and ask them to private message you to provide the solution. This helps prevent a "SCREAMING" argument from them in response. If there's a simple solution, politely talk to them in the reply offering a solution. Many times, a Yelp reviewer will update or delete their bad review when it's been answered immediately. In other words, keep your cool! If they delete or update their review, it can have a positive effect on your SEO.