There’s no way to directly update the cable, but it should update automatically when plugged into your Mac. While you can check the firmware of your power adapter by attaching it to your Mac and clicking on the Apple logo in the top left corner of the menu bar, then About This Mac, System Report, and Power, it’s not clear how to check the specific firmware version of the cable.
So presumably there is some UART to the cable, and the cable to the power adapter. Is it all stuffed in the cable treated as a USB device or is there some proprietary control on the laptop side to communicate with the cable?