The steam client updates will be downloaded automatically and will be installed the next time Steam is restarted.

From the Steam Powered News Page:

A Steam client update is now available. To apply the update, click the File menu inside of Steam and then select “Check for Steam Client Updates…”. The specific changes include:

Steam Client

* Fixed NAT-traversal library using too much CPU during L4D – matchmaking
* Fixed several localization issues
* Fixed install wizard forcing clients to install the full version of a game instead of the demo version, when the full version isn’t released yet
* Added l4d.com to the whitelist of chat url’s not to warn about

Tags: steam client update, steam powered