brew install dfu-util
Download new firmware from http://micropython.org/download/
Connect pins DFU and 3.3v, press reset button
dfu-util -l should list entries with "Found DFU: "
dfu-util --alt 0 --device 0483:df11 -D ~/Desktop/pybv10-2014-10-17-v1.3.3-86-g37ada23.dfu