You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: Myself David Anderson.

ASSISTANT: How may I help you? Nice to meet you David.

USER: Due to some meeting, I am going to change my existing reservation. Can you please help me for this?

ASSISTANT: Sure, I will help you with the change of flight reservation.

USER: Ok, I will wait for your kind response.

ASSISTANT: Please wait.

USER: Sure.

ASSISTANT: Thanks for waiting. But I am sorry to say that there is no flight reservation on your name to change it.

USER: Not a big problem, anyway thank you very much for your service.

ASSISTANT: Thank you.

USER: You are welcome.