Thanks.
I figured out how to start the update.
When typing explicitly ‘php consolidation/robo/robo …’ it works but just plain ‘robo’ doesn’t work and htis redirect form vendors/bin/ isnt working on my Windows machine.
P.S. you need php to be on your classpath for this to work.
Not exactly the way it should work but the upgrade gets started this way and also terminates successfully.