I'm using Mac OS X and the default tex editor is TexWorks, but I want to utilize TexShop as the default one.
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Assuming you're on Mac OS X (otherwise there would be no TeXShop available to you), all you need is to click on a
In the window that pops up go at the tab named "Open with" and from the popup menu choose "TeXShop.app"
Then click on the button just below, named "Change All…"