After read this, I go to WordPress.org to download new version, than I just follow this instructions

Upgrade 2.0.3 to 2.0.4

This is a security upgrade. Information can be found in the WordPress Dev Blog 2.0.4 Announcement (http://wordpress.org/development/2006/07/wordpress-204/).

As with all WordPress upgrades, the contents of your /wp-content/ folder should remain intact and unchanged.

1. Delete /wp-admin/ folder.
2. Delete /wp-includes/ folder. Note: You must backup /wp-includes/languages/ directory if exists.
3. WordPress 2.0.4 contains an updated default theme. If you would like the updated theme, delete the /wp-content/themes/default folder. Remember to make a backup of your customisations first!
4. Delete all the wordpress files in the root directory where the root index.php file is found. DO NOT DELETE wp-config.php (nor any files you may have added yourself: robots.txt, .htaccess, favicon.ico).
5. Download and extract the new version.
6. Upload it to the appropriate folders such as /wp-admin/, /wp-includes/, and if you deleted your Default theme, /wp-content/themes/default/.
7. Upload all the files in the root directory.
8. If existed, restore /wp-includes/languages/ directory
9. Login as admin into your site – you will not be prompted to click on the link to upgrade your database as there were no changes to the database scheme between 2.0.3 and 2.0.4.

You can read more detail here and done