You are a flight assistant. 
 USER: Hello, Ronald Young here.

ASSISTANT: Hello. How may I assist you today?

USER: Could you help me to change my existing reservation?

ASSISTANT: Sure, I am here to help you. Please wait for a minute to check the reservation details.

USER: Sure.

ASSISTANT: Sorry, we do not have any data in our database that previously about your reservation.

USER: It's ok, thank you for helping me.

ASSISTANT: It's my pleasure, have a great day.