These are the default key bindings:
The way it is by default doesn’t make sense considering we have only a few buttons. The lightkey module only repeats keys that are used with the shift key on the front. LK3 it’s only a shift button, which we already have at the front.
So, is there a way to put completely new key bindings to the lighkey module (apart from shift+LK1,LK2,LK4 & LK5)? and why does it come with this key bindings by default? 
As far as I know the controller unit is considered a key pad which means buttons have assigned letters (keypad= mix of gamepad and keyboard?). There is no in OS way to modify those. The only thing you can do is switch the y/x and a/B positions in the settings menu to switch between the xbox and snes layout. Those controls will carry over in the navigation menu and you can apply them to retro arch. The shift key on the gamepad is not the same as the shift key on LK3. You see the shift key on the game pad actually only is the number 0 as the LK3 is an actual shift key in terms of computer imput
Edit: I have not tested this but pressing and holding the shift key on the game pad may switch between the inputs that are devided by the /
You can change the layout (and also implement new features) if you reflash the keypad controller, this is the code repository: https://github.com/clockworkpi/Keypad/blob/master/clockworkpi_keypad.ino
Its easily done with arduino IDE
I’ve just saw you post @Petrakis .
Unfortunately I don’t have any idea how to do this. I’m stuck in trying to install the CH340 driver. I downloaded the linux .zip that comes with a Makefile and the ch34x.c file, but I can’t install it.
make[1]: Entering directory '/usr/lib/modules/4.20.3-1-MANJARO/build'
make[1]: *** No targets specified and no makefile found. Stop.
make[1]: Leaving directory '/usr/lib/modules/4.20.3-1-MANJARO/build'
make: *** [Makefile:5: default] Error 2
An even after that, I don’t have any idea on how to flash the arduino board… 