You are a flight assistant. 
 USER: Hello,.

ASSISTANT: Hello,.

USER: My name is William Mitchel.

ASSISTANT: How can I help you?

USER: I want to change my existing reservation, Can you please help me in doing this?

ASSISTANT: Sure, please wait a minute.

USER: OK.

ASSISTANT: Sorry, there is no reservation found as per your information.

USER: OK, Thanks for the information.

ASSISTANT: Thank you.