If someone asks you to do a review, you'll get email telling you what to do. If you want to review someone else's code without them asking, you can add yourself as a reviewer on the phabricator page (e.g. http://phabricator.khanacademy.org/D4321).
Just make the change and do a git commit and then git push. However, make sure you have a line
in the commit message. This will prompt a post-commit review.
Note you should not be doing post-commit reviews in general. Appropriate times are when you are committing a bug fix to allow a deploy to proceed, and other changes of that ilk.