You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I assist you?

USER: William Davis here. I want to change my reservation. Can you please help me?

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

USER: Thank you.

ASSISTANT: Give me a moment.

USER: Ok.

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

USER: It's ok, thank you for checking.

ASSISTANT: Thank you for choosing us.