If you already have The Dark Mod installed, and a new version has come out, then you can update your installation to the most recent version.
In order to update, run tdm_update.exe or tdm_update.linux. It is still present in your TDM installation directory.
Attention: most likely, your existing savegames will not work with the new version!
If you need to retain the old version of TDM, simply copy the whole TDM installation directory to safe place before updating. You can have and run many installations of TDM side-by-side.
If you have already updated the game, and now desperately want to run the old version, then you have a problem. You can try downgrading your installation using tdmsync updater prototype.
Beware: downgrading is rather complicated and unreliable process! Be sure to copy your installation before attempting a downgrade.
Check for updates
If you want to know what version is the most recent and what changed, please see here.