You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how may I help you?

USER: I want to change my existing reservation. Could you help me?

ASSISTANT: Sure, we are here to help you. May I know your name please?

USER: Margaret Clark.

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

USER: That's ok, no problem. Thank you.

ASSISTANT: Thank you for contacting us.