Even though I couldn’t get it to run through the openhabian-config, @omr was so kind to explain to me how it could be done manually
cat /etc/apt/sources.list.d/openhab2.list
shows the old content. Change the file (e.g. with nano) to have the following content
deb http://openhab.jfrog.io/openhab/openhab-linuxpkg unstable main
After changing file, do:
sudo apt-get update
sudo apt-get upgrade
What could have worked though in openhabian config is to go into 40 openhab-related and then switch in 41 to the unstable version. Haven’t tried that though because the manual way did work…