To edit your site after you have published it, log into the client area, and click Edit site from the clientarea dashboard. You will be taken to the Sitebuilder's Editor. Make your changes in the editor and Publish the site again. Any changes you have made to the site will immediately appear on the live site after you have clicked on the Publish button.
Note: if you do not see the changes you have made after you have clicked on the publish button, it is probably because your browser is displaying an older version of the page from its cache. Just reload the page using your browser's reload button to bring up the newest version of your site.