You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am Barbara Williams.

ASSISTANT: Barbara, how can I assist you today?

USER: I want to change my travelling dates to my existing reservation. Can you help me to change my reservation?

ASSISTANT: Sure, let me check.

USER: Ok.

ASSISTANT: Sorry Barbara, I have not found any reservation under your name.

USER: Ok. Thank you for your search.

ASSISTANT: Thank you for contacting us, have a nice day.