You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How may I guide you today?

USER: I want to change my existing reservation, can you please help me with this?

ASSISTANT: Sure, I am glad to help you with that.

USER: Thank you.

ASSISTANT: Can I know your name to check your previous reservation?

USER: I am Joseph Thomas.

ASSISTANT: Please be on  hold.

USER: Sure.

ASSISTANT: We regret to inform you that, there is no reservation found on your name in our reservation list.

USER: Ok, no problem. Thank you for your information.

ASSISTANT: Thank you for reaching us, have a great day.