The original post is a little ambiguous in it's phrasing and could be interpreted as wanting to change the projection of EXISTING drawings OR just to change the template for NEW drawings to be 3rd angle.
I had this problem too in Inventor 2013
Basically you need to have a drawing open (1st or 3rd angle) otherwise the menus relating to drawings are not available.
Then, go to the Manage tab, select Styles Editor and then you can change the settings in your chosen standard.
NB. In Inventor 2013 you can change the default template at the Welcome screen (Welcome button on the Get Started tab)
So change your style to what you want at startup (Get Started) eg. ISO mm, then go to Styles Editor and change the settings for that Style to what you want, eg. Third Angle Projection and then Save the style to the drawing. Then go to the Manage tab, and hit Save to save Styles from the current document to the Style library.
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