I just updated my AirPort Extreme to the lastest firmware (7.6.1). Since then I have had really poor performance. I would like to downgrade to the previous firmware, but I can't figure out how to do it with the new GUI. Before 7.6.1 you simply opened up the manual setup and clicked on "version". That is not how it works with the new GUI. If you click on "version" in the pop-up window it does not open another window, it just changes the firmware version number from 7.6.1 to the exact build number. I have clicked on "edit", but there is no option that I can find to downgrade from there, either. I don't even see a setup assistant. How do you downgrade the firmware using the new GUI?
Assuming your using Airport Ulilty 6.0, which in all accounts is a "Lite" version. Try downloading and install Airport Utility 5.6, both can be installed. This should put you back to the old interface and allow you to do what you want.