You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I help you?

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

ASSISTANT: Sure, I can help you with that and can I know your name for checking purpose?

USER: My name is Robert Garcia.

ASSISTANT: Thank you for your information, please give me a moment to check your previous booking.

USER: Sure.

ASSISTANT: Thank you for your patience, I regret to inform you that there is no reservation found for the user name in our back end to change your ticket booking.

USER: Ok, that's not a problem. Thanks for your information.

ASSISTANT: You are most welcome and have a nice day.