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You change this by logging in as administrator, going to system > preferences > general settings and there is a text box with this option:
Reason for turning off the site
The text that is presented when the site is closed.
You enter in the text box the text you want and save the changes.
You can also change the design of this page by opening the system_siteclosed.html template found in this directory:
modules > system > templates > system_siteclosed.html
You add your design to it, but then you don't upload it to the same place. You save it inside: yourthemefolder > modules > system > system_siteclosed.html
(you have to create the "modules" folder and "system" folder inside your theme folder)
This template overriding feature allows you to update your modules and core files without worrying of losing changes done to the templates, because the templates will remain in your theme folder, it also helps designers alot in making template sets.
If you do this you have to set to "Yes" the option (in system > preferences > general settings) :
Check templates for modifications ?
If this option is enabled, modified templates will be automatically recompiled when they are displayed. You must turn this option off on a production site.
So your changes can take effect and no cache is created. When you finish doing changes to the templates you set this option back to "No".