You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I assist you?

USER: David Perez here, due to an emergency, I would like to change my existing reservation. So could you please help me for that?

ASSISTANT: Sure David, hold on for a moment. Sorry to say this David, we did not found any reservation ticket under your name.

USER: Ok, thank you for your information.

ASSISTANT: You are welcome.