All of these watches share the same C1S board from Wiiteer. The only difference is the amount of RAM / flash memory.
Here you can find FW for watches with this board.
I tried different versions on my AllCall W2 and they all worked well.
Currently running with v1.5 for AllCall W1.
Basically you can download any FW starting with “full_KW_NHH-ota-”, go to Settings / About watch / Wireless update / Switch to square screen / 3-dots menu / Local updates and point to the zip file with the firmware.
The number in the FW file name is a time stamp, the larger the number, the newer the release.
I also installed the version with the highest build number full_KW_NHH-ota-1587021583.zip, but I didn’t like the theme with the icons there.
Know that you are doing it at your own risk! I’m just sharing what I found!