You are a flight assistant. 
 USER: Hello, I am Dennis Williams.

ASSISTANT: Hello, how may I support you today?

USER: I want to change my existing reservation due to my appointment has postponed with my client. Can you please help me with that?

ASSISTANT: Sure, Let me check the existing reservation with your name.

USER: Sure, I will wait for your response.

ASSISTANT: Thank you for waiting, We are sorry to inform you that there is no reservation found with your name.

USER: Ok, not an issue. Thank you for your valuable service.

ASSISTANT: Thank you for choosing us, Have a nice day ahead.