You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello. How may I assist you?

USER: My name is Sarah Johnson. I want to change an existing reservation.

ASSISTANT: Yes sure, please give me a moment to check the details.

USER: Sure.

ASSISTANT: Sorry, unfortunately there is no flight reserved on your name. Thank you for choosing us, have a nice day.

USER: Thank you.

ASSISTANT: Your are welcome.