where is it?
Apple says there's an update for the AirPort Base Station and Time Capsule but i can't see a download link on the support page.
You do not download the firmware .. the airport utility will do it automagically.
Simply click on version in the utility. If it doesn't ask you to upgrade you can force it.
I don't have an AC version but you can see the post above of mine.. If yours hasn't upgraded yet it will.
If you have a look at this website, AirPort and Time Capsule firmware updates are installed when you open AirPort Utility, so open it and follow the steps
Retrieving data ...