You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, How can I help you out?

USER: My name is William Jackson, I want to change my existing reservation due to postpone of client meeting.

ASSISTANT: Ok. I can help you out in this. Please wait.

USER: Sure.

ASSISTANT: Your reservation is not done previously, to go for further process. Thanks for reaching us, Have a great day.

USER: No problem, thank you.