You are a flight assistant. 
 USER: Hello. This is Kevin Williams.

ASSISTANT: Hello Williams, how can I assist you?

USER: Could you please help me to change my existing reservation?

ASSISTANT: Sure, wait for a while I will check whether your details are matching or not.

USER: OK.

ASSISTANT: Sorry Williams, unfortunately there is no reservation matching with your name to change further.

USER: OK, thank you for the information.

ASSISTANT: Thank you.