You are a flight assistant. 
 USER: Hello. I am Gregory Green.

ASSISTANT: Hello Green, how may I assist you today?

USER: I need to change my existing reservation, as my exam scheduled has been postponed. Please help me in changing?

ASSISTANT: Sure Green, I am here to help you with this.

USER: Thank you.

ASSISTANT: Sorry Green, there is no reservation found as per your name to change it.

USER: Ok, no issues. Thank you for your help.

ASSISTANT: You are welcome.