Re-install firmware via download - how-to?

As you can see from my other comment (app can't connect), I am currently unable to retrieve that information from any of my three devices.
The trouble started after a recent firmware update. Initially, my site and its devices could still be found, but switching to bridge mode would consistently (every time I tried) first show success, but when entering that config screen again, it would report NAT mode, i.e. no success.
I have gone thru several attempts at re-setting the devices and starting from scratch. Things just got worse. Now, there's one device that's still offering the "correct" WiFi, but the other devices can't be configured any more.