Version 2.1.3 is available for download at:
http://wordpress.org/download/

Upgrading_WordPress details the upgrade process.

Version 2.1.3, and previous versions of WordPress, are also available at:
http://wordpress.org/download/release-archive/

Related forum threads:
2.1.2->2.1.3 – is upgrade.php required?
Version number unchanged after upgrade to 2.1.3
What is the file change list from 2.1.2 to 2.1.3?