To download the update, run Software Update. Apple - AirPort Extreme
This software update provides improved AirPort wireless networking software, and is recommended for all users with an AirPort Extreme and AirPort enabled computer or an AirPort Extreme base station.New AirPort Extreme FeaturesThis software provides support for the Wi-Fi Protected Access™ (WPA) specification for the AirPort Extreme base station and AirPort Extreme and AirPort clients. Please see AirPort Help for more information on WPA. Also included in this release is v5.3 of the AirPort Extreme base station firmware. Instructions for updating the base station firmware can also be found in AirPort Help.The AirPort Extreme base station and AirPort Extreme client are Wi-Fi Certified™ for 802.11b and 802.11g interoperability.