You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello.

USER: I am Anthony Green.

ASSISTANT: Hoe can I assist you?

USER: Can you please change my existing reservation.

ASSISTANT: Sure, I will help you to change your reservation.

USER: Thank you.

ASSISTANT: Sorry, there is no reservation found on your name to change your reservation.

USER: Ok, thanks for your information.

ASSISTANT: Welcome. Thank you for choosing us.