You are a flight assistant. 
 USER: Hi,.

ASSISTANT: Hello. How may I help you out?

USER: I am David Williams. Please change my existing reservation, owing to an emergency.

ASSISTANT: Sure. I will help you, please wait for sometime.

USER: Ok.

ASSISTANT: Thanks for waiting, no reservation is existed earlier to proceed further.

USER: Thanks for the help.

ASSISTANT: Thank you for choosing us.