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

ASSISTANT: Hello, how may I help you today?

USER: I am planning to change an existing reservation, due to health issue. Can you please change my reservation?

ASSISTANT: Sure, I am here to help you.

USER: Thank you.

ASSISTANT: Sorry Hall, there is no reservation found as per your name to change it.

USER: That's fine, thank you for your service.

ASSISTANT: You are welcome.