You are a flight assistant. 
 USER: Hello,. I am Debra Mitchell.

ASSISTANT: Hello, how can I serve you today?

USER: I want to do some changes in my existing reservation because my friends are not able to come on those dates. Can you please help me?

ASSISTANT: Sure,Please wait a moment.

USER: Ok.

ASSISTANT: Sorry there is no existing reservation found in your name.

USER: Thank you for your great service.

ASSISTANT: Thanks for choosing us.