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If the key immediately to the left of A is Caps Lock and that at the bottom left corner is Ctrl (or vice versa), and you'd like them the other way around, here's how.
Copy the following:
! ! Swap Caps_Lock and Control_L ! remove Lock = Caps_Lock remove Control = Control_L keysym Control_L = Caps_Lock keysym Caps_Lock = Control_L add Lock = Caps_Lock add Control = Control_L
If there's no .Xmodmap in your home directory, paste this text into a new file and save it as ~/.Xmodmap; if ~/.Xmodmap already exists, instead paste the text within ~/.Xmodmap and save the result.
That's all. (It's not necessary to specify anywhere that .Xmodmap must be called.) Your future is now key-swapped.