Set primary monitor for a dual monitor setup?

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http://forums.fedoraforum.org – Here is a screenshot: (I'm not sure why there is a black rectangle at the top of the screenshot) Right now, the red monitor is primary and the green monitor is the secondary (where the desktop extends onto). How could I reverse this? I meant to make the laptop monitor primary and the additional monitor secondary, the monitor that the desktop would extend onto. Any assistance is appreciated, thanks! (HowTos)