You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How may I aid you today?

USER: I want to change my existing reservation which is done recently due to some problems, can you please help me?

ASSISTANT: Sure, I am glad to help you to change your reservation.

USER: Thank you.

ASSISTANT: Could you please tell me your name?

USER: Sure, my name is Margaret Miller.

ASSISTANT: Sorry, no reservation found on your name to change.

USER: Ok, that's fine. Thank you for your service.

ASSISTANT: Welcome, thank you for checking.