You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I help you?

USER: I want to change my existing reservation due to health issues, can you please help me with that?

ASSISTANT: Let me know your name.

USER: My name is Gregory Moore.

ASSISTANT: Reservation found. Can I know your airport codes?

USER: My source and destination airport codes are PHL and IAD.

ASSISTANT: Let me know your travelling dates.

USER: I want to fly on June 13, return on June 15.

ASSISTANT: Do you have any preference?

USER: My depart time should be afternoon. And I want a flight with one connection.

ASSISTANT: Sorry, no flights found for your travelling dates.

USER: No problem, thank you.

ASSISTANT: Thank you.