Hold the Boot button - the button with "BOOT0" near usb-c port - while the board is reset (either by connecting USB or by pressing reset button). Release the Boot button shortly after the board has reset. The board ought to now be in DFU mode and detectable as such from a connected computer.
Use a tool like dfu-util to update the
firmware:
dfu-util --alt 0 -D firmware.dfu
These are automatic builds of the development branch for the next release.