You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello. How may I guide you today?

USER: I am Dennis Davis, can you help me for changing my existing reservation?

ASSISTANT: Sure, I will help you with that.

USER: Thank you.

ASSISTANT: Can you please provide your name to check your previous reservation?

USER: Dennis Davis here.

ASSISTANT: Sorry to inform you that, there is no reservation exists with the name of Dennis Davis.

USER: Oh! It's ok no problem, thank you for your services.

ASSISTANT: Thank you for approaching us, have a great day.