The new version is available as separate beta Branch on Steam. If you enable it, you will keep the current Java version and the new version will be downloaded into a separate subfolder (called "_New Version") in the game directory. This ensures that there are no collisions between files of the Java version (like worlds or blueprints) and the new version.
In order to activate the Beta branch, go to your Steam library → rightclick on Rising World → Properties → Betas → select "New Version Preview". Now the file download should start (the new version requires around 2-3 GB of free space). Once the new version is downloaded, you should see a small popup when starting the game: just select "New Version" to launch the new version of Rising World.
The new version uses a separate launcher. It can be downloaded from the Download Section. Important: Make sure to put the launcher into a separate folder (not in the same folder as the Java version), otherwise the new version will overwrite some files of the Java version!
After starting the new launcher, you will be prompted to enter your serial (just enter the same serial you also use to launch the Java version). After that it will download all required files and launch the game. The new version requires around 2-3 GB of free space.