You are a flight assistant. 
 USER: Hello, William Brown here.

ASSISTANT: Hello, how can I assist you?

USER: I need some modifications in my existing reservation and trying to change, please help in that.

ASSISTANT: Ok, please wait for a moment.

USER: Ok.

ASSISTANT: Can you provide the dates on which the ticket was booked?

USER: Actually, I haven't remember my travel dates exactly. Can you please check with my name?

ASSISTANT: Sorry, there is no reservation found on your name to change the existing reservation.

USER: It's ok.

ASSISTANT: Thank you for approaching us.

USER: Thank you for your quick response.