You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello.

USER: Myself David Lewis.

ASSISTANT: Lewis, how can I guide you today?

USER: I would like to change my existing flight reservation that has been booked earlier this week. Could you please help me in re-modifying my dates in the reservation?

ASSISTANT: Sure, I will help you. Please wait for a while.

USER: Sure.

ASSISTANT: Sorry Lewis, I did not find any reservation under your name in our database.

USER: It's ok, anyway thank you for the information.

ASSISTANT: It's my pleasure, have a nice day.