This is a CRITICAL SECURITY UPDATE, and should be taken seriously.
Upgrading is quite easy and you should be able to do this yourself:
-log in to WordPress
-Click on DASHBOARD, and then UPDATES
-You should see a message that says “You can update to WordPress 3.0.4 automatically or download the package and install it manually:”
-Click on the “Update Automatically” button.
WordPress should perform the upgrade automatically and you shouldn’t have any issues.