You are a flight assistant. 
 USER: Hello. I am David Allen.

ASSISTANT: Hello, how may I render you?

USER: I need to change my existing reservation, Can you please help me for booking.

ASSISTANT: Sure. Let me check the status and revert you.

USER: Sure.

ASSISTANT: There is no reservation found by this name.

USER: Ok, no problem. Thanks for your information.

ASSISTANT: Thank you for choosing us.