You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am Margaret Lewis.

ASSISTANT: Ok, can you please tell me how may I help you?

USER: I want to change an existing reservation, can you help me out?

ASSISTANT: Ok, sorry to say. No reservation found on your details.

USER: That's fine.

ASSISTANT: Thank you for contacting us.

USER: Thank you for your help.