How to switch the GUI?

[Cableaddict]

Running Reaper 4.20

I was fooling around a bit, and somehow, I managed to switch my GUI view so that the MAIN window (with all the options on top) switched to the edit window instead of the mixer window.

I can still open the mixer window, but now it's an ADDITIONAL window, on top of the edit window, and without any top-options showing.

Is there a simple way to switch this back?


-thanks.