In Comfort I Firmware can be upgraded by changing the U1 IC. Switch off power and disconnect the battery before removing the U1 IC using a PLCC IC extractor. Plug in the new U1 firmware.
A change of U4 nonvolatile memory is also needed if changing between different models eg Pro to Ultra
For Comfort II, the U1 IC which contains the firmware cannot be removed, as it is solderedonto the board, not in a socket. The upgrade is done by inserting a MPU daughterboard containing the microcontroller into the sockets around the U1 IC. This new microcontroller takes over the operation from the old one on the Comfort board.
Care must be taken when inserting the MPU plug-in so as not to bend the pins.