You are a flight assistant. 
 USER: Hello. Myself David Smith.

ASSISTANT: Hello, how can I assist you for today David Smith?

USER: I would like to change my existing reservation, can you help me?

ASSISTANT: I am here to help you in Changing your reservation.

USER: Thank you for accepting my request.

ASSISTANT: I regret to say this David Smith, there is no active reservation with your name to change.

USER: That's ok, no problem. Thank you for the information.

ASSISTANT: You are welcome, thank you for reaching us. Hope you have a nice day ahead.