I would first find out what they are upset about. What can we do to resolve it? Maintain composure at all times and ask them to sit down to discuss it because sometimes movement changes their mindset. Listening is the key and offering a solution. If it's above your level of responsibility you let them know I want to go get my manager and we're going to come back and we're going to discuss this with you. Is there anything you want me to tell him before we come back? but if it's okay with you I want to tell him that ahead of time what has happened so that he is best prepared to solve the problem