How do I revise a copyright?

You must file a new copyright. Copyrights are for the work of original authorship that is submitted and registered.

Any work that qualifies for copyright protection gains that protection the minute it is fixed in some sort of permanent medium. So both your work and the revisions are already copyrighted (assuming they are your original work). If you choose to, you can register your work with the U.S. Copyright Office in the Library of Congress. Submit the whole revised work to the Copyright Office, even if you already submitted an earlier version, so there is no question about what what is protected.