I think that you need an ST-Link Adapter like this
-STM32 STM8 Programmers & Debuggers ST-LINK/V2 (mini)
-BrainPad Arduino Code Programming via the serial SWD Interface
be careful to watch the picture. The blue wire is GND!
Edit: ST-Link Adapter is not needed. Bootloader and Firmware can be actualized as usual (see post below). I only had forgotten how it is done.