Download the file pack found at http://core.trac.ww.wp.xz.cn/changeset/22201. Do so by clicking the “Download in other formats: Zip Archive” at the bottom, or clicking the individual files in the list near the top.
Take those files and replace the existing files found in your Twentytwelve theme directory.
If you’re looking to use a child theme you’ve got a bit more work ahead of you. You’ll need to look through the difference (red = old file, green = new file) that patch made in the Twentytwelve files and integrate them (by hand) into your child theme. As always, take a backup and be prepared to use it.
