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: Sure, can you share your name for checking?

USER: My name is Rachel Lee.

ASSISTANT: We have found your reservation, can you share your trip dates?

USER: I want to fly on July 20, return on July 22.

ASSISTANT: May I know your airport codes?

USER: My source and destination airport codes are JFK and ATL.

ASSISTANT: Do you have any preferences?

USER: My depart time should be afternoon. And I need a single connecting flight.

ASSISTANT: Sorry, there is no flight on your given dates.

USER: No problem, thank you.

ASSISTANT: Thank you for reaching us.