Firmware update 7.7.2 is for AirPort Extreme and AirPort Time Capsule base stations with 802.11ac.
● Addresses an issue that prevents a USB hard drive connected to an AirPort Extreme from appearing as an available disk in Time Machine
● Resolves an issue that may intermittently cause a loss in WAN connectivity
● Resolves an issue that may result in a network loop
All about AirPort Time Capsule, AirPort Extreme, and AirPort Express base station firmware updates: http://support.apple.com/kb/HT1218.
You can install the update by launching AirPort Utility from your Applications:Utilities folder.