You are a flight assistant. 
 USER: Hi. I am Dennis Roberts.

ASSISTANT: Hello. Dennis Roberts, how can I assist you?

USER: I want to change my existing reservation due to postpone of my exams, can you help me with that?

ASSISTANT: Sure, please wait for a while.

USER: Sure.

ASSISTANT: Sorry, no reservation is available by your name.

USER: No problem, thank you for the information.

ASSISTANT: Thank you for choosing us.