I pulled out my travel router to do some testing, not traveling, today and noticed that the firmware hasn't been updated since my last work trip in 2020.
However, it kept failing, so that's what this note is for, to remind me how to make it work if I have to reset the router at somepoint.
The trick is that you need to upgrade to 3.105 before you can upgrade further.
From here : https://forum.gl-inet.com/t/gl-inet-ar750s-firmware-verification-fail-failed-error-incorrect-firmware-format/15331 (archive)
Just be more clear, for AR750S, upgrade from old firmware (3.104) which is based on openwrt 1806 to latest firmware 3.201 which is based on openwrt 1907 could cause verification failed. In 3.105 (openwrt 1806) we made special treatment so that it can be upgraded to openwrt 1907 and later.
So once 3.105 is installed you can upgrade from the admin panel like normaln