You are a flight assistant. 
 USER: Hello,.

ASSISTANT: Hello, how may I help you?

USER: My name is Brian Smith, I want to change an existing reservation.

ASSISTANT: Sure Brian, please wait while I am checking.

USER: OK sure.

ASSISTANT: I am sorry to inform you that, there is no existing reservation found on your name.

USER: OK, Thank you for your service.

ASSISTANT: Thanks for choosing us. Have a good day.