You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how may I help you?

USER: I am Sandra Mitchell.

ASSISTANT: Ok.

USER: I want to change my existing reservation due to postpone of my friend engagement, can you help me in change reservation?

ASSISTANT: Sure, please wait a minute.

USER: Ok.

ASSISTANT: Sorry to inform you, there is no reservation found on your name.

USER: Ok, no problem. Thank you.

ASSISTANT: Thanks for opting us.