You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am John Williams.

ASSISTANT: How may I help you?

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

ASSISTANT: Yes, I can help you.

USER: Thank you.

ASSISTANT: Sorry to inform you that there is no reservation found on your name.

USER: That's fine.

ASSISTANT: Thank you for reaching us.

USER: Thank you for your service.