You cannot change the Supernova difficulty. The most difficulty difficulty level in The Outer World is called Supernova, and includes extra gameplay elements like the fact that companions can permanently die, or having extra survival elements that you need to deal with. In addition to this, the enemies are even more difficult than in “Hard” difficulty.
A unique thing about the Supernova difficulty is that you cannot change it at any point, so all games that are started on this difficulty must also be finished on it. If you find it too difficult, all you can do is to start a new game.