This board can programmed via DFU bootloader, using e.g. dfu-util.
To enter the DFU bootloader, double tap the reset (blue) button, or you can use machine.bootloader()
from the MicroPython REPL.
dfu-util -w -a 0 -d 2341:035b -D build-ARDUINO_GIGA/firmware.dfu
Or from MicroPython source repository:
make BOARD=ARDUINO_GIGA deploy