To answer both of your questions:
You send your transcripts to LSAC. Once LSAC has ALL of your transcripts, they will process a report. When you apply to a law school, the law school will request your report from LSAC. You simply need to pay the fee for the report ($10-12, depending on when you sign up). When you have updated transcripts, you need to send them into LSAC. I'm not positive about this and couldn't find it in two minutes on LSAC, so you should e-mail LSAC, but I believe they automatically send your updated report to the schools that have already received your report. Again, check on this.