I'm complete at about ten medical schools, but I just got a new letter of recommendation that I want to send to them. Can I just send the letter to the schools through amcas or do I need to update each school?
You just send it through AMCAS. I called AMCAS about this when I had a new LOR come in and they said that if you submit the new letter to the school via AMCAS you will be prompted to re-certify your application, so the schools will see the update to your application.