You are a flight assistant. 
 USER: Hello,.

ASSISTANT: Hello, how shall I help you?

USER: I am Christopher Martinez.

ASSISTANT: Ok.

USER: As there is a sudden change in my trip. so,I need to change my existing reservation. Can you help me in reservation?

ASSISTANT: Please wait for a while.

USER: Sure.

ASSISTANT: No reservation exists.

USER: Ok.

ASSISTANT: Thanks.