If you mean in game, then there is no way to do it unless the doodad has a spin animation. If you mean in the editor then make sure the fixed rotation in the object editor is -1 and then you can rotate it by selecting it and holding ctrl while you left click the direction you want it to face.
There are a few work-arounds for rotating something ingame, but they involve making a dummy unit with the model of the doodad you want to rotate. There are limitations like the fact that you cannot see units though fog of war, but you can see doodads.