Ok. I figured it out with the
Can not connect to target! message.
Thanks to mayaku in this post
viewtopic.php?p=54736#p54736
You don't need another ST-LINK V2 (as I bought). You need just to reset controller on Pin 7 with ground from ST-LINK. After reset I was able to do all. Maybe you will have message, that there is read out protection.
Disable Read Out Protection and retry.
In order to unprotect the chip with ST-LINK, one has to click Target -> Option Bytes: -> change Read-Out Protection - Disabled.
Then it is possible to switch to System memory (0x1FFFF800) and see firmware binary:
Flashing is possible now.
Thanks a lot and enjoy the solution
Greetings