Got the instructions from @jacobacci
Get current Volumio image
I used Balena Etcher to flash over a MacBook
Connected a monitor to the HDMI (just to see what is going on)
Connected mini-USB to MAC and push the boot bottom on the backside.
Balena Etcher found the Computer Module right away
Select source and destination for the volumio image
let it cook for some time - (might need a cold restart after 60min, mine did)
If you do a shutdown in the Volumio GUI the Pro-Ject Stream Box S2 Ultra will not completely shutdown, you need to push the power bottom on the front too.