How to deal with a bad review?

Keep calm. Take a negative review as an opportunity to shine if you manage it correctly. Negative reviews are the most read. Users pay special attention to them. Take them seriously and as a improvement to better your business. Never take it personal. Be polite and focus on solving the problem.
Users can be mean and rude, but keep cool and calm. Do not to respond impulsively, be professional. Once you have provided a solution, you have the possibility to request your customer to change his/her review. If your customer changes the review it will be shown instead of the initial review. However, the chain of messages will be shown afterwards, so you can show your management abilities.