you can find the source code for version 2.3.9 athttps://github.com/Warzone2100/warzone2100
click on releases and then keep on going until you find the version you want and download it.
Once it's downloaded extract the tar.gz file into a folder in your home directory (for example warzone2100).
Open up the terminal window (I use Ubuntu so a ctrl-alt-t shortcut works for me) and change to that directory (cd warzone2100)
then type ./configure into the terminal. This should check to see if you can compile it and will tell you what packages you need to install if you're missing them (sudo apt-get install <missing package name>).
Once ./configure runs through with no error messages, then you can go to the next step which is to type:
sudo make install
into the terminal window. This will compile the game and may take a few minutes. Once it's completed then type
into the terminal window and that will start the game. Afterwards the game should start simply by typing warzone2100 into the terminal window.
The flavour of Linus I use is Ubuntu, so some of the commands for installing the packages may differ depending on the version of Linux you use, so remember, google is your friend.