By default, the game is limited to Beginner only. If you purchase the Pro version of the game, you will have the possibility to change the difficulty from Beginner to Intermediate, Expert and 24x30 tiles.
This is specified on the official website in the FAQ section:
Different presents are available from the File menu. Available presets are 8x8, Beginner, Intermediate, Expert, and 24x30. You can also customize the board size and number of mines. Note that Minesweeper 101 is limited to Beginner difficulty only. Upgrade to the full version to play other presets like Intermediate, and Expert, and to custom sizes.
