A few steps to upgrade WordPress
1. Backup WordPress database with phpmyadmin or WordPress Database Backup plugin
2. Deactivate any plugins
3. Delete files and folders, except wp-config.php files and wp-content folder
4. Upload the latest WordPress, except wp-config.php files and wp-content folder
5. Type in domain_name/wp-admin/upgrade.php or domain_name/wp-admin on your browser to run the upgrade command, just follow the instructions
6. Activate any plugins
Note : WordPress 2.8+ and above provides an easy way to upgrade WordPress via wp-admin.
Important! you should backup your database and files first before you do the backup