The update process is very simple, and complete instructions are available here. Note that there are now easier and better ways of updating than copying the files with FTP.
Download
- New Installations: Click here to download Joomla 2.5.21 (Full package)
- Update Package: Click here to download Joomla 2.5.21 (Update package)
- Note: Please read the update instructions before updating.
Discuss the release: http://forum.joomla.org/viewtopic.php?f=9&t=853578