Apple has released AirPort Base Station Firmware Update 7.6.4 which is available for all Apple 802.11n Wi-Fi base stations including AirPort Express, AirPort Extreme, and AirPort Time Capsule.
It contains fixes that:
• Resolve an issue that may cause AirPort Time Capsule to report that the backup disk is in use
• Resolve an issue that may cause an IPv6 Tunnel error
• Resolve an issue that may intermittently cause a loss in Wi-Fi connectivity with iOS devices
For information on the security content of this update, please visit: http://support.apple.com/kb/HT1222
For detailed information on AirPort base station firmware updates, including instructions how to install the update, please visit: http://support.apple.com/kb/HT1218