Updating the firmware of the PSLab v6 the way it is decribed on https://github.com/fossasia/pslab-firmware did not work for me. I used mcbootflash instead, which is installed as a dependency of the pslab Python library:
./mcbootflash --port /dev/serial/by-id/usb-Silicon_Labs_CP2102N_USB_to_UART_Bridge_Controller_5ecc207eeba8eb11828e98374232452f-if00-port0 -b 460800 ~/Downloads/pslab-firmware_v6.hex
The ID is individual for each board.