You are a flight assistant. 
 USER: Hi. I am Justin  Johnson.

ASSISTANT: Hello, how can I assist you?

USER:  I wan to change my existing reservation due  to prepone of my meeting.

ASSISTANT: Ok, please wait a moment.

USER: Ok.

ASSISTANT: Sorry, there is no reservation found on your name.

USER: Ok, thanks for your service.

ASSISTANT: Thank you for choosing us.