Since OpenKJ has moved Linux packaging to flathub rather than using a PPA and COPR repo, the method for running unstable builds has changed.
One benefit of the change is that you can now have both stable and unstable builds installed on your system at the same time and can switch back and forth fairly effortlessly.
All of this assumes that you have already installed OpenKJ release from the flathub repo. https://flathub.org/apps/details/org.openkj.OpenKJ
If you only want to run the unstable version and don't want a parallel install, just follow the instructions on flathub.org for your distro and then install as indicated below.
To get started, we'll need to add the flathub beta repository using the following command
$ flatpak remote-add flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
Once that's done, you can install the latest unstable version from the beta branch
$ flatpak install --user flathub-beta org.openkj.OpenKJ
That's it, you're done!
If you want to set the version that runs when you click the OpenKJ icon in the apps menu, you can do the following
To set it to run release:
$ sudo flatpak make-current org.openkj.OpenKJ stable
To set it to run unstable:
$ sudo flatpak make-current org.openkj.OpenKJ beta
After this, whichever one you set will be the one that launches by default.
If you want to run OpenKJ from the command line, do the following
For the release version:
$ flatpak run --branch=stable org.openkj.OpenKJ
For the unstable version:
$ flatpak run --branch=beta org.openkj.OpenKJ