How to unlock the Early Access Steam achievement by reverting to a previous version of the game.
Reverting and Unlocking
Since current game version doesn't allow the achievement to pop, it will be needed to downgrade the game version to a previous one, in which it works.
To start doing so, type Win+R (to open windows execute program) and type steam://open/console while steam is open.
In the steam console, type "download_depot 344040 344041 5285719388050115027" (this only works for windows x64).
Steam will download a package, which probably will be located in C:\\program files(x86)\Steam\steamapps\content\app_344040\depot_344040 (there will be clear hints in the console you opened earlier to where it will be located). Inside it, when the download is finished, will be an older version of the game, in which achievements works, i tested it myself.
From there, just erase all files within C:\\program files(x86)\Steam\steamapps\common\Voxelized (or wherever you installed your game), and copy the contents of the previous folder (C:\\program files(x86)\Steam\steamapps\content\app_344040\depot_344040), and paste all of them to your game folder in steamapps\common\Voxelized.
Launch your game from steam and achievement will unlock, and a whole another game will reveal itself.