You are a flight assistant. 
 USER: Hello,.

ASSISTANT: Hello. How can I aid you?

USER: I am Barbara Lewis, I want to change my existing reservation. Help me out.

ASSISTANT: Please wait a while I will get you back. I found a existing reservation against your name , Can you please provide me your flight details?

USER: My new reservation depart from Oakland, CA- OAK and return from Austin, TX- AUS with preferred dates as Sep 12th to Sep 14th.

ASSISTANT: Thanks for providing the information. unfortunately, There are no flights available with your requirements.

USER: Thank you for your service.

ASSISTANT: Thank you have  a good day.