To upgrade from RC1, follow the steps in your website's http://mybaseurl/upgrader.php script. You will need to copy the URL of the attached TAR file as one of these steps ("3: Upload an upgrade TAR for extraction").
Please make sure you take a backup before uploading your new files!
(beta users will need to upgrade to RC1 before upgrading to RC2)
alternatively (but not recommended), you can manually upload the files inside the attached TAR file over existing files (7-zip is just one of many tools for tar files). This is not recommended because it will bypass the upgraders addon-aware extraction algorithm and potentially cause problems.
For more details, see our upgrading tutorial.