To enter the bootloader, press and hold the 'Octave +' and 'Octave -' buttons while powering SL MKIII. The bootloader will display a dim green bar on the key LEDs, and some of the drum pads and soft buttons will be red.
The lit pads represent the firmware version number in binary coded decimal as 4 characters with 4 pads per character. The lit soft buttons display the same for the bootloader version number.
During a firmware upgrade, the key LEDs will display a moving green progress bar, followed by a moving orange progress bar. It is important that you do no power off SL MKIII while the orange bar is showing. It is normal for the bar to pause for a few seconds in the middle of the process.
When SL MKIII has valid firmware installed, the play button will be green. Press this to launch the firmware.