Setting a single part in a 2d array

You know how
myArray=[[0,1,2,3],[4,5,6,7],[8,9,10,11]]
trace(myArray[2,1])
will give you 9?

Well, how do I edit ONLY the number in myArray[2,1]?
Like, I thought it would be something like myArray[2,1]=2419, but weird stuff happens then and I think it overwrites my old myArray