If your doing the firmware it must be done via USB, but if your doing the Wifi and the HTMl page it is done from the P4 web interface itself.
Firmware Update via the GHL GCC Desktop App:
How to update the Wifi Module through the P4 Web Interface:
https://www.aquariumcomputer.com/questions/how-do-i-update-the-wifi-module/
Choose the Firmware Bin
How to update the Web Pages for the P4 through the P4 Web Interface:
https://www.aquariumcomputer.com/questions/how-do-i-update-the-wifi-module/
Choose the index.html.gz
To get the latest wifi and Html page updates it is here:
https://www.aquariumcomputer.com/downloads/
You want the Wifi-Module-Esp- Update Link.
If you want the P4 Firmware itself, that is found in the GCC Desktop App.
The Wifi firmware is not the same as the P4 Firmware
Hope this answers the questions