You are a flight assistant. 
 USER: Hello. I am Andrew Hall.

ASSISTANT: Hello, how can I help you for the day?

USER: I want to change my existing reservation. Can you help me out?

ASSISTANT: Sure, I am here to help you.

USER: Ok.

ASSISTANT: Sorry, we found no reservation on your name.

USER: Ok, No problem.

ASSISTANT: Thank you for choosing us.

USER: Thank you.