From here: https://askubuntu.com/a/101877
Create a file called .Xmodmap
in your home directory.
$ vim ~/.Xmodmap
Add the following lines to the file:
clear control
clear mod1
keycode 37 = Alt_L Meta_L
keycode 64 = Control_L
add control = Control_L Control_R
add mod1 = Alt_L Meta_L
Save the file. Next time you login the new key mappings will be active. To have the settings take immediate effect run the command:
$ xmodmap ~/.Xmodmap
There's a chance that the keycodes above are different on your computer. If the above doesn't work, try running the xev program from the command line. Xev will generate a new small window in the corner of your screen. If you start pressing keys on your keyboard you’ll get all the key mapping information for that key press.