Rummy (HD)
Tap on the small button with the arrow pointing down and then the gearwheel. The menu that appears now has two tabs: "Rules" to customize the game rules and "Game" to make general game settings.
Tap on "Game" to set the difficulty. You can choose between "easy", "normal" and "hard". By default "normal" is selected.
Rummy Multiplayer
Press the button with the three lines in the upper left corner and select "Settings". Under Offline you can, among other things, adjust the difficulty. You can choose between "easy", "normal" and "hard". By default, "normal" is selected.