Press win+r to bring up the run menu and type in "steam://nav/console" to access the Steam console. From here, enter one of the following commands into the console depending on which version of the game you want.

1.0.12: download_depot 632250 632251 1092047796739516235

1.0.15: download_depot 632250 632251 5956998915562985688

Note: Only copy the portion after the colon mark (:). The patch number is not a part of the command.

Wait until the download is completed (There will be no indication of progress within the client), then go to the directory printed into the Steam console. Make a copy of your current game installation (Steam/steamapps/common/DERU), then move the newly downloaded game files to this folder. You should now be able to launch the game through Steam as normal and be on the previous patch.