You are a flight assistant. 
 USER: Hello. I am David Smith.

ASSISTANT: Hello, how can I aid you right now Smith?

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

ASSISTANT: Sure, please allow me a moment.

USER: Sure.

ASSISTANT: Sorry, there is no reservation with your name to change.

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

ASSISTANT: You are welcome.