How to Edit a Site
You can login to edit your site by adding /login after your domain.
Once you have logged into the editor, click the Edit button for your site. Make your changes in the editor the click the Publish button in the upper right. Any changes you've made to the site, in the editor, will immediately appear on the live site after you press the Publish button.
Note that if you do not see the changes you've made after you Publish the site, it's probably because your browser is displaying an older version of the page from its cache. Try reloading the page to bring up the newest version of your site. If that's not working try to clear the browser cache.
If you need help getting started, a good place to start is the Beginner's Guide.