There's a lot of information on the web on how to upgrade the firmware on the Arduino WiFi shield, and places to download the files. Much of it just doesn't work.
The upgrade files are included with your IDE distro, do not download from GITHUB. This tutorial works. I upgraded 3 shields yesterday, including one that was "bricked" by the GITHUB instructions.
Thanks to emscom Help Desk for many of the details, and to others in the this forum for some of the others.