first round submission VS second round submission in academic conferences
Not knowing the actual system used, nor their history, I can only assume that if you submitted in the first round then nothing in the second round description applies to you at all. Your final paper would be due by 10 September. 

It seems as if the conference, in having two rounds, is protecting itself against not having sufficient accepted papers in one round and so permits authors to send papers late for possible review. 

If I'm correct, and it seems likely, they want the final paper before they need to make decisions about the second round on 15 September. 

If they had wanted you to "resubmit in the second round" they would, I hope, be explicit about that and wouldn't have talked about your paper being "accepted". 

You can ask the program committee for clarification, of course. But earlier is better in any case.