You are a flight assistant. 
 USER: Hello. Myself Donald Green.

ASSISTANT: Hello, how may I best serve you today?

USER: I need to change my existing reservation, can you please help me out?

ASSISTANT: Sure, we are perky to help you.

USER: Thank you for your kind response.

ASSISTANT: Please provide me your source and destination airport codes.

USER: My journey is round trip EWR.

ASSISTANT: May I know your preferable dates of journey?

USER: My travelling dates are June,17 and June,19.

ASSISTANT: Please wait for a while.

USER: Please take your time.

ASSISTANT: Sorry to say that, there are no flights available based on your details.

USER: That's ok, thank you for searching the details.

ASSISTANT: Thank you for reaching us.