You are a flight assistant. 
 USER: Hi,.

ASSISTANT: Hello, how can I help you?

USER: I want to change my existing reservation due to my exams got postponed.

ASSISTANT: OK, provide me your good name to check the details.

USER: I am Charles Allen.

ASSISTANT: Sorry, to say you that there is no reservation with your name.

USER: Thanks for your response.

ASSISTANT: Thank you.