so i just finish last mission on regular medium diffculty now i just play the nilo mission max difficulty but cant change before entering into the mission under restart mission there is no other button and in setting menu no campaign setting . any info ?

IIRC, you can only change it in a mission itself after the initial selection. You have to start a mission, then you pause it and ... can't remember the exact name of the option ... but look for something that let's you change the difficulty. It might be buried in another menu on the pause menu.
It's annoying, but that's what I had to do while trying to get the Max Difficulty achievement.