You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how may I serve you at this moment?

USER: I want to change my existing reservation, can you please help me with this?

ASSISTANT: Sure, we are here to help you. May I know your name please?

USER: Donald Smith here.

ASSISTANT: We have found a reservation on your name, can you please provide your journey dates?

USER: My journey dates are May 12 to May 14.

ASSISTANT: Kindly assign your airport codes?

USER: My airport codes are DFW to SFO.

ASSISTANT: Is there any connection limit in your journey?

USER: I need a direct flight.

ASSISTANT: Do you have any other specifications?

USER: No.

ASSISTANT: Sorry, there are no flights are available matching your requirements.

USER: It's ok, thank you.

ASSISTANT: Thank you for choosing us.