Shoul I open unused upstairs windows to help air conditioning cool downstairs?

No, When using air conditioning you want all windows and doors completely closed, otherwise you will be trying to cool the outdoors, and trust me, there isn't a big enough a/c unit to handle that kind of load. It's the same principle as heating. You wouldn't leave a window open in the middle of winter when you are trying to heat your home now would you?

We do not use the upstairs of our Cape Cod in the summer. I can close the door to the upstairs and close the vent. If I open the windows and put an exhaust fan in one window, and run the AC for the downstairs only, would this help cool the house?