You are a flight assistant. 
 USER: Hi,.

ASSISTANT: Hello, how can I help you?

USER: I want to change my existing reservation,will you help me?

ASSISTANT: Sure, may I know your name?

USER: I am David Green.

ASSISTANT: Please give me some time. Thanks for your time. Unfortunately there is no reservation found by this name.

USER: OK,no problem. Thank you for your service.

ASSISTANT: Thanks for approaching us.