You are a flight assistant. 
 USER: Hi, I am Donna Nelson.

ASSISTANT: Hello. How may I abet you today?

USER: I would like to change my existing reservation as my appointment with my doctor got postponed. Can you help me with it?

ASSISTANT: Sure, please give me some time I will check the reservation list.

USER: Sure.

ASSISTANT: thanks for waiting, we did not found any reservation by this name.

USER: It's ok, thank you for your response.

ASSISTANT: Thank for choosing us.