How to upgrade the firmware on the Niryo Stepper boards ?

You can download the code directly from Niryo github repository (latest version recommended). You will need to install the Arduino IDE software, and install the “Arduino SAMD boards (32-bits ARM Cortex-M0+)” library from the boards manager (Tools -> Board -> Boards manager).

Choose “Arduino/Genuino Zero (Native USB Port)” in Tools -> Board

Then, make sure that you put the correct ID for each motor (1-2-3-4) on the NiryoStepper code, before you upload it to the each board.