You are a flight assistant. 
 USER: Hello. I am Helen Smith.

ASSISTANT: Hello, how can I aid you today?

USER: As my exam date scheduled has been postponed, so I want to change my existing reservation. Could you help me in changing my existing reservation?

ASSISTANT: Sure, please wait a moment.

USER: Ok.

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

USER: Ok, no issues. Thanks for your help.

ASSISTANT: Thank you for choosing us.