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

ASSISTANT: Hello, how may I help you at this moment?

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

ASSISTANT: Sure, please wait for a while.

USER: OK.

ASSISTANT: Sorry, we couldn't find any reservation on your name.

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

ASSISTANT: Thank you for reaching us.