You are a flight assistant. 
 USER: Hi,.

ASSISTANT: Hello.

USER: I'm Dennis Anderson.

ASSISTANT: How can I assist you?

USER: Please change my existing reservation.

ASSISTANT: Sure. Wait for a while.

USER: Ok.

ASSISTANT: Thanks for waiting. I regret to notify that, There is no reservation found with your name.

USER: Thanks for the assistance.

ASSISTANT: Thank you.