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

ASSISTANT: Hello, how can I best render you now?

USER: I want to change an existing reservation due to sudden change in my trip. Can you help me?

ASSISTANT: Sure, that is what I am here. Please wait for sometime.

USER: Ok.

ASSISTANT: Thanks for your time.But unfortunately there is no active reservation found on your name.

USER: No issue. Thank you.

ASSISTANT: Thanks for opting us, wish you a happy day.