You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how may I serve you?

USER: I want to change my existing reservation. Could you help me with that?

ASSISTANT: Sure, I will help you. Please provide the name on which ticket was reserved.

USER: John Clark.

ASSISTANT: Sorry, there is no reservation existing on your name.

USER: That's ok, no problem. Thank you.

ASSISTANT: You are welcome, have a nice day.