Your desktop environment probably has a way, but you don't say which one you're using (if any).
If your display driver is compatible with the XRandR extension, which is the standard X.org method for managing display resolutions and arrangements, you can use the command-line utility
xrandr. I think the proprietary NVidia driver bypasses XRandR, so if you're using it, you'll have to use a dedicated NVidia tool.
xrandr (with no argument) to see your monitor (screen) arrangement. You'll see lines like these:
DVI-0 connected 1600x1200+1600+0 (normal left inverted right x axis y axis) 408mm x 306mm
DVI-1 connected 1600x1200+0+0 (normal left inverted right x axis y axis) 408mm x 306mm
This example means that I have two monitors called
DVI-1 is at the top left (position
DVI-0 is to its right (position
+1600+0). To swap them, I would run
xrandr --output DVI-0 --left-of DVI-1