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Upgrading From A Previous Version

To upgrade from a previous version of this plugin, delete the entire folder and files from the previous version of the plugin and then follow the installation instructions below.

Installing The Plugin

Extract all files from the ZIP file, making sure to keep the file structure intact, and then upload the plugin's folder to /wp-content/plugins/.

This should result in the following file structure:

- wp-content
    - plugins
        - wordpress-admin-bar
            | jquery.checkboxes.pack.js
            | readme.txt
            | screenshot1.png
            | wordpress-admin-bar.js
            | wordpress-admin-bar.php
            - themes
                - blue
                    | blue.css
                    - images
                    [...]

Then just visit your admin area and activate the plugin.

See Also: "Installing Plugins" article on the WP Codex

Installing The Plugin For WordPress MU

Install normally, but move just wordpress-admin-bar.php to mu-plugins. Leave the rest of the files in the normal place.