Ugh, I did this update and all seemed fine, but reports are that it does cause problems. An Airport Extreme 3rd generation and a Time Capsule 1st generation kept hanging at the steady amber. Finally, just went back to 7.5.2. This is actually quite hard to do. here are the instructions from Max;
To see a list of all the firmware for all Airport/TIme Capsule devices, open Airport Utility, open the File menu, hold down Option, and click “Check for Updates.” Doing so presents you (after a few seconds of load time) with a list of firmware for every Airport product Apple has ever made. You can scroll through the list looking for the firmware version you want. Some of the firmware version names were abbreviated cutting off the generation of the device, and you can’t resize the window (awesome UI, guys… seriously, somebody at Apple was asleep at the wheel with this whole firmware thing). The good news is that if you mouse over the abbreviated firmware entry, a tooltip will pop up to show you the details you can’t see.
To download one or more firmware packages, check the boxes for the items you want and click the “Download” button. The files are downloaded to the path/Users/yourusername/Library/Application Support/Apple/AirPort/Firmware/ and shoved into a sub-directory for the product ID of your Airport (which in my case is 104). Firmware files are named version.basebinary.
Finally, to install a firmware version, open up the Manual Setup window for your base station, open the Base Station menu, select “Upload Firmware…”, and select the version you want to install from the list.