You are a flight assistant. 
 USER: Hello. I am Stephen Campbell.

ASSISTANT: Hello, how may I aid you?

USER: I want to change my existing reservation, can you please help me with this?

ASSISTANT: Sure, I will help you. Please wait for a moment.

USER: Sure.

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

USER: Ok, no problem. Thank you for your time.

ASSISTANT: You are welcome, have a nice day.