There are four difficulty modes in Iron Marines Invasion: Casual, Normal, Veteran, and Impossible (which is unlocked after completing the campaign in any difficulty mode).   

To choose your difficulty mode

From the Map, go to the Settings menu by tapping on the gear icon on the bottom left. 

On the top left, find the Difficulty settings. It will display your current difficulty level.

Tap it and choose your preferred difficulty mode. 

You'll now see the new difficulty you selected under difficulty

To check the max difficulty that you completed on a planet, access the holodeck and check the rank marks.