WordPress.org

You can use the built in installer and upgrader, or you can install the plugin manually.

1. You can either use the automatic plugin installer or your FTP program to upload it to your wp-content/plugins directory the top-level folder. Don't just upload all the php files and put them in /wp-content/plugins/.
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Visit your SEO options (Options - All in One SEO) for Wordpress 2.3.x, (Settings - All in One SEO) for Wordpress 2.5.x-2.8.x
4. Configure any options as desired, and then enable the plugin
5. That's it!

If you have to upgrade manually simply repeat the installation steps and re-enable the plugin.

If upgrading, please back up your database first!

Upgrade guide for upgrading from 1.5.x and below to 1.6.x