You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I assist you?

USER: My name is William Williams.I need to change my existing reservation.

ASSISTANT: OK, I will check your details and revert you.

USER: Sure.

ASSISTANT: Sorry for the inconvenience, we did not found a reservation on your name.

USER: OK, Thanks for your information.

ASSISTANT: Thank you for contacting us, have a good day.