You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am Patricia Walker.

ASSISTANT: How may I help you?

USER: As my trip to New York has postponed, so I want change my existing reservation. Can you please help me out?

ASSISTANT: Sure, wait a moment.

USER: Ok.

ASSISTANT: Sorry, there is no reservation has been found. Thank you for approaching us.

USER: That's ok. Thank you for your time.