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.Apex settings, Home screen settings, Manage screens. There you simply drag the yellow star to the screen you want to use as default home screen.
I'm not using Apex at the moment, but can't you set it from the preview screen?

I think you misunderstood the question.
Apex IS the default launcher (in fact, I uninstalled the one that came with my ROM). My problem is that of the three home screens, the one that it defaults to when I hit the Home button is not the one I want it to, and wading through the launcher's settings hasn't helped. The developer's website is spectacularly unhelpful as well.

I had not realized the star was draggable.
On the same topic - when I turn on the phone it comes up with the last screen opened. How do I start on the home screen?