You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello,.

USER: I am Sandra Turner.

ASSISTANT: How can I render you?

USER: I want to change my reservation due to prepone of my appointment with doctor. Can you help me in my changing reservation?

ASSISTANT: Sure, wait for a while.

USER: Sure.

ASSISTANT: Sorry, there is no reservation exists in your specified name.

USER: Not a problem. Thank you for your help.

ASSISTANT: Thanks for approaching us.