- Backup your existing website (files and db)
- Download the latest Wordpress zip file, and extract on your local machine
- Access your site via FTP and remove/delete/rename (whichever suits you) these 2 directories in your current installation wp-admin and wp-includes
- Upload the new wp-admin and wp-includes from the newly extracted zip file
- Then upload the rest of the files overwriting the existing ones
- Check your admin site, WP might ask you to update your database.
That's it.