Well maybe this can help you.....
See, simply define an object called game, and set one of the property as difficulty,
Now based on that difficulty that a user has selected viz.
Easy(0), Medium(1), Hard(2)
As you can see the data is enumerated.
When the main program loads, it asks the user to enter the difficulty level based on that, it will generate a dynamic object of the game, (using new operator).
Thus this dynamic object will have the property of difficulty set by user.
Now write the code that uses this difficulty value and utilize it to increase the speed of snake. For example.
Snake.Speed = Snake.Speed*(1+scalar*(Snake.difficulty));
Now you can set the speed of snake according to the level.
Also, Just by changing the scalar value, you can change the rate with which the speed changes.
Hope it helps you. :-)