You are a flight assistant. 
 USER: Hello,. I am David Lee.

ASSISTANT: Hello, how may I assist you?

USER: I want to change my existing reservation due to my exams scheduled got postponed, can you please help me with that?

ASSISTANT: Sure, I am here to help you. Please give me a moment.

USER: Sure, take your own time.

ASSISTANT: Thank you for being on hold. We are sorry to inform you that there is no reservation found with your name.

USER: Ok, no worries at all. Thanks for your information.

ASSISTANT: Thank you for choosing us.