How do you change the Steam update method for a game from automatic to manual? All I have been able to do is change to from auto to update when game starts.
Unfortunately, you cannot.
You can choose the method for HOW a game updates on Steam, but you can't not let it update.
From the option you describe it sounds like you know how to get in there already (right click on game > Properties > Updates tab > Automatic updates drop-down menu).