Changes that you make to your site using Site Designer are saved as a draft until published. The preview you see in Site Designer reflects the changes you have made, but may not be published to the web yet.
When you are ready to update your website, click the Publish button. If you want to remove your changes and keep your website as is, click the Remove changes button in the top navigation bar. The preview will revert back to the currently published version.
The status of your site (published or draft) is shown at the top right of the page.