You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am David Lewis.

ASSISTANT: How may I guide you today?

USER: I want to change my existing reservation due to postpone of my exams, can you help me with that?

ASSISTANT: Sure, I will help you with that. Please wait for a minute.

USER: Sure.

ASSISTANT: I am sorry to say that there is no reservation found under your name.

USER: No issues, thank you for the information.

ASSISTANT: Thank you for reaching us.