To swap control and command, add the following to your ~/.Xmodmap file:

remove control = Control_L
remove mod4 = Super_L Super_R

keysym Control_L = Super_L
keysym Super_L = Control_L
keysym Super_R = Control_L

add control = Control_L Control_R
add mod4 = Super_L Super_R

The above current works for both an Apple laptop and an Apple keyboard.

Also, to use Command+Tab to cycle windows, make sure to update keyboard shortcuts to use Ctrl+Tab instead of Alt+Tab (for Xfce, Window Manager -> Keyboard). All other keyboard shortcuts should come through (from the Control to Command swap perspective).

Source: https://stackoverflow.com/questions/7099602/cmd-control-keys-swap-in-ubuntu