Hi!
Basically you just need to replace all the Moodle scripts with the new versions, then visit http://yoursite/admin and everything else will be upgraded smoothly. You shouldn't lose anything.
Just in case something goes wrong, you should always make a backup of the site first, or test it on a copy of the site.
Full information: http://docs.moodle.org/en/Upgrading
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Installing Moodle 1.9.2 for security updates
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