You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello.

USER: Barbara Carter here.

ASSISTANT: How can I assist you?

USER: Due to some health issue, I want to change my existing reservation. Can you please help me?

ASSISTANT: Sure, I will help you to change your reservation. Can you wait a while?

USER: Ok.

ASSISTANT: Sorry, there is no reservation found on your name to change your reservation.

USER: It's ok, thank you for checking.

ASSISTANT: Welcome.