N3DS Stella (Atari 2600) - swap joystick port

Hello all How can I change the joystick port control in Retroarch Stella for N3DS? I mean how to swap joystick ports and have the 3DS pad emulating the right/left joystick. A number of games (Air-Sea Battle, Basketball…) seem to be working by default with joy 2 which makes them unusable in 1-player mode vs. Computer variations.