You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How may I  help you today?

USER: I would like to change my existing reservation. Please help me out.

ASSISTANT: Sure, I am happy to help you. Can I know your name?

USER: Sure, my name is Jennifer Mitchell.

ASSISTANT: Yes Mitchell, your reservation is found, provide me your airport codes?

USER: Sure, my airport codes are IAD and SFO.

ASSISTANT: Sure, can I know your travelling dates?

USER: My planned dates are 07/13 and 07/15.

ASSISTANT: Ok, wait for a minute.

USER: Sure.

ASSISTANT: Sorry to say that, no flight's available based on your travelling dates.

USER: Ok, no problem. Thank you for your information.

ASSISTANT: Thank you for approaching us.