At the Main menu screen highlight the Marathon option and hold down the Left Shoulder Button and press Select. On the Marathon game selection screen there will be a new game option called Popular, this is the classic version of Tetris.
At the "Main Menu" screen, select "Marathon." At the choose "Game Type" screen, press the L and Select buttons together. The "Popular" selection will appear highlighted at the bottom of the screen. Select it to play a classic version of Tetris.