1. Make a backup (Backup guide)
  2. Download the lastest version.
  3. Unzip the downloaded file.
  4. Upload all files to your hosting or local server via FTP, Shell, Cpanel, others.
  5. With your browser, go to the URL /update.php. Example: www.domain-name.com/update.php
  6. Delete the file update.php
  7. If you use “friendly urls” youn need update the file .htaccess
  8. Check e-mail accounts settings on Dashboard→Settings→Notifications are complete and correctly configured
  9. Done!

If something fail, you can delete all files from your hosting or local server and make a clean installation, and then with the backup replace the folders on your clean installation.