From the DOW 3 fourms.

"Here is how you publish update...
1) Identify and delete the steam id folder of your mod ( its a folder with a number like 11233232 something ) its created in same place as yoru local .sga file
2) Save your work so the worldbuilder or Mod tools editor saves your current version to the local folder.
3) Now start the game and do not i repeat do not open multiplayer tab if you do by accident redo from step 1.. Now after your game starts click Community tab select your mod and hit publish.. it will ask for update change long text if you have it type somehting you can edit later in steam.
4) After your done publishing quit the game go to local file again and delete the steam folder that you deleted in step 1.. and wait for a couple of minutes before starting the game. and the next time you start the game this steam number folder will have latest copy and you can play your version in game to test..

------------Here is how to test your updates before publishing your mod------------
1) save your update from worldbuilder or Mod Tools.
2) Identify the respective steam number folder in local for your mod DONT DELETE IT.. copy the saved .SGA file from step 1 to this steam id folder.
3) Start the game and you should have your updated mod for testing.."