You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello.

USER: I am William Robinson.

ASSISTANT: How may I aid you?

USER: Can you help me for changing my existing reservation?

ASSISTANT: Sure, I will help you with the change of flight reservation.

USER: Thank you.

ASSISTANT: Thanks for waiting. But I am sorry to say that there is no reservation on your name to change it.

USER: Oh! I think my sister already changed my reservation, anyway thank you for your services.

ASSISTANT: Thank you.

USER: It's my pleasure.