If you were reviewed by the first board but did not receive a scholarship you can update your school list, and your SAT/ACT score, that's it. You can not make any other updates or changes to your application once it has been reviewed by a board.
If you have just submitted your application and it was not reviewed by the first board then you have until the deadline for the next board to make any changes you like. Just log into your application and make the changes. Make sure you receive confirmation from Cadet Command.