According to Apple, the 7.7.2 update fixes the following:
- When a USB hard drive connected to an AirPort Extreme does not appear as an available disk in Time Machine;
- An issue that causes a loss in WAN connectivity;
- An issue that causes a network loop.
To install the update, you need to use the AirPort Utility on a Mac, which is usually installed in the Applications > Utilities folder. Launch the app, and it will list the AirPort base stations on the network and any updates that are available. Then select the device, and click the Update button. You can also perform the update using the Airport Utility iOS app on an iPhone, iPad, or iPod touch.
Apple offers a support document on installing AirPort base station firmware updates if you'd like more information.