You can find all released firmware versions in the archive directory.
To install a firmware file, first install the teensy-loader-cli Debian
package, then run e.g.:
teensy_loader_cli -w -v --mcu=TEENSY36 fw-2018-03-24.hex
…and press the physical programming mode switch on the kinX board.
On Debian stretch, use:
apt install make gcc-arm-none-eabi
mkdir Debug
(cd Debug && make -f ../Makefile)
Follow the Installation section above for how to install the resulting .hex file.
mkdir Debug
docker run -v $PWD:/usr/src/mk66f-fw kinxproject/mk66f-fw bash -c 'cd /usr/src/mk66f-fw/Debug && make -f ../Makefile'