Kibana packages up files into the root level /optimize directory before serving them to the browser. If your server isn't running in development mode, or the files in the optimize directory aren't updated, then your changes won't be visible.
https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#development-environment-setup is a good resource for getting a dev environment setup.
If you have downloaded a packaged version of Kibana (off https://www.elastic.co/downloads/kibana for example) and want to make a quick change without setting up a dev environment, deleting the contents of the optimize folder will cause the server to rebundle everything when it next starts and changes should be visible.