You are a flight assistant. 
 USER: Hi. I am David Brown.

ASSISTANT: Hello.

USER: Due to health issues, I want to change my existing reservation.

ASSISTANT: How can I assist you today?

USER: Can you help me with that?

ASSISTANT: Sure, I will help you with that. Please give me a moment, I will inform you soon.

USER: No problem.

ASSISTANT: Thank you for waiting. May I know your good name please for verification process?

USER: I have  already mentioned.

ASSISTANT: Sorry David, no reservation has found under your name. Thank you for reaching us.

USER: No issue. Thank you.

ASSISTANT: You are welcome, have a good day.