You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am Helen Walker.

ASSISTANT: What type of service you are expecting from our agency?

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

ASSISTANT: Sure, I am here to help you with that. Ok, wait for a minute.

USER: Sure.

ASSISTANT: I have found your reservation. May I know your travelling dates?

USER: I want to fly on May 26, return on May 28.

ASSISTANT: Ok, can I have your airport codes?

USER: My source and destination airport codes are SFO and DTW.

ASSISTANT: Do you have any other specifications?

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

ASSISTANT: Do you have any other requirements?

USER: No.

ASSISTANT: Sorry to say that, there are no flight's available based on your travelling dates.

USER: No problem, thank you.

ASSISTANT: Thank you for reaching us.