The method described below should be used for relatively minor modifications, such as adjusting menus or quickly reorganizing content sections. More complex tasks, such as installing new components or adjusting complex configuration settings should be performed and tested on a development server first. Not only does this keep your public site up and running, but it also lets you test at your leisure, thus reducing errors. One way to do it is to create a sub-domain (i. e., dev.yourdomain.com) and install Joomla! there just as it is installed on your public site.
1. Login to the administrator section, and choose: Site > Global Configuration.
2. The first option you'll see is is to set the site offline. Choose "Yes" and press the Save button. This will hide prevent display of all site pages, and replace them with the following message:
"This site is down for maintenance. Please check back again soon. message instead."
3. While you are logged into the "back end" administrator system, you can still view the "front end," by choosing Site > Template > Preview. This will display the site as it would appear to users along with a warning at the top that the site is down for maintenance.