You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I help you?

USER: I am Kevin Clark. I want to chanhge my existing reservation, due to some health issue. Can you please help me?

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.

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

ASSISTANT: Welcome, have a nice day.