It’s under world settings, usually world settings are on the right side of the editor, if not go to window - world settings. If you made a game mode blueprint already then it is inside of that. I am confused as to why you want to use the third person template, but delete everything out of it. That is the same thing as creating a completely blank project. If you use a blank template, then from inside of that open a new level, and select basic empty, it might achieve the result you want.
Edit; Ok, reading it again it sounds like what you want to do is change the third person characters mesh? To do that, go to the third person blueprint, and select mesh in top left. In the details panel, change the skeletal mesh asset to the new one.