You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello. How may I assist you today at this moment?

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

ASSISTANT: On which name your ticket has been reserved?

USER: My name is Mark Thomas.

ASSISTANT: Please give me a moment.

USER: Sure.

ASSISTANT: We are sorry to inform you that there is no reservation found with your provided name.

USER: Not an issue, thanks for checking.

ASSISTANT: Thank you for availing us.