You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how may I be at your service today?

USER: Can you please help me to change my existing reservation?

ASSISTANT: May I know your name please?

USER: Sure, I am Donna Walker.

ASSISTANT: I am sorry no reservation found on your name.

USER: Ok, no problem. Thank you for checking.

ASSISTANT: Thank you for choosing us.