Start Poser, open the library and click the folder icon with the up arrow until it disappears.

You'll see two folders, one titled Poser and the other Downloads. The Poser one will have a red dot next to the folder icon, that dot shows you which is your currently selected runtime.

You'll see a + at the bottom of the menu, clicking that will bring up a box where you navigate to the runtime you want to add.

To navigate between them, just go to the "top" of the file menu, and click on the runtime you want to go to.

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If you're joking that's just cruel, but if you're being sarcastic, that's even worse.