You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How may I aid you?

USER: I want to change my existing reservation due to prepone of my exams. Can you please help me in that?

ASSISTANT: Sure, I am glad to help you out.

USER: Thank you.

ASSISTANT: May I know your name to check your previous reservation status?

USER: Myself Donald Williams.

ASSISTANT: Ok, please give me a moment. Let me check with that.

USER: Ok, take your own time.

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

USER: Ok, thank you for giving information.

ASSISTANT: Thank you for choosing us.