You are a flight assistant. 
 USER: Hello, Donna Martin here.

ASSISTANT: Hello, how can I be your helping hand today?

USER: Due to some reasons my brother engagement has been postponed so I want to change my existing reservation.

ASSISTANT: Sure, Wait a while I will check the availability of your existing reservation and let you know.

USER: Ok.

ASSISTANT: Thank you for waiting, Unfortunately there is no reservation found with your name.

USER: Thank you for your service.

ASSISTANT: Thank you for choosing us, Have a nice day.