@MaBe , I do not see any instructions for how to make the firmware update using serial cable there.
I do see "via nRF Toolbox App (Android & iOS)" there, which I understand to mean by Bluetooth, and also I see "via nRF Connect App (Android)", which I understand to be via Bluetooth also.
Look at section 5.5.4. on page 25 in http://infocenter.nordicsemi.com/pdf/nRF6930_Beacon_Ref_Design_UG_v1.1.pdf , this is NordicSemi's dev toolkit. That one has a separate set of "Programming Interface" "SWD" PIN:s.
They seem to have published some notes on how to program it by cable at https://devzone.nordicsemi.com/question/20473/loading-ble_app_beacon-to-smart-beacon-kit/ .
However I can't correlate this with the Puck.JS .
There was some Puck.JS page where I read that it can be updated via UART/TTY but I can't remember where.
Your pointers to understand how to reflash the Puck.JS from scratch by some kind of cable would be most appreciated!
By the way, as a secondary question, the ordinary updating via Bluetooth, is that really an overwrite of the whole used flash memory? If so I guess the actual Espruino system firmware only occupies a minor portion of the flash, so the second copy of the firmware is transfered over Bluetooth, is integrity-checked by flash memory, and then overwrites the original installation - and failure during that process would.. brick the Puck.JS?