You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I help you?

USER: Jack Wilson here. 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, take your time. Sorry to say, there is no reservation on your name in our database.

USER: It's ok, thank you for your help.

ASSISTANT: Thank you for opting us.