If you want manual in Simulation mode change it from the main screen in "Options". Its in there somewhere. For arcade mode... just change it when u select ur car, which im sure u knew
still, spend about 45 seconds clicking around the "options" menus, wher eyou would logically expect this sort of thing, and it is hard to miss. Even a trained chimp could find it. Hell a normal chump could find it I am sure.