Usually you boot your S6 into Download (or Odin) Mode by shutting it down and pressing and holding the Power, Home and Volume Down Buttons. But in my case Odin is not able to connect to the phone. The only way that works for me is by using
adb reboot download
When the phone is running, USB Debugging must be enabled in the Developer Settings and I have to set the USB connection to MIDI Device, otherwise
adb is not able to connect.
On related news: I now run Android 7 via Lineage OS.