Diamond1: Difficulty is shown in the options screen. You can adjust it there (Normal -> Vet, Vet -> Normal, Iron Man -> Normal or Vet, but you can't up it to Iron Man from Vet or Normal)
Maxvorstadt: Nope, in the options screen there is nothing about difficulty. It doesn`t show the current difficulty and there is also no option to change the difficulty.
I remember there being an option to reduce the difficulty, but not to increase it.
(This is actually a problem, as this is one of those games with an inverted difficulty curve, meaning a player might want to increase the difficulty of the game once they're used to the game and are passed the initial difficulty hump, but the game won't let them.)