Setting elements in a matrix

I am sorry, but I can't understand why I am not able to change the 
elements of a square matrix. I tried the following

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

(1) -> m : SquareMatrix(2,INT) := [[1,2],[3,4]]

         +1  2+
    (1)  |    |
         +3  4+
                                                    Type: SquareMatrix(2,Integer)
(2) -> m(1,1):=0

    The form on the left hand side of an assignment must be a single
       variable, a Tuple of variables or a reference to an entry in an
       object supporting the setelt operation.
(2) -> setelt(m,1,1,0)
    There are 2 exposed and 0 unexposed library operations named setelt
       having 4 argument(s) but none was determined to be applicable. Use
       HyperDoc Browse, or issue
                                )display op setelt
       to learn more about the available operations. Perhaps
       package-calling the operation or using coercions on the arguments
       will allow you to apply the operation.

    Cannot find a definition or applicable library operation named setelt
       with argument type(s)
                             SquareMatrix(2,Integer)
                                 PositiveInteger
                                 PositiveInteger
                                NonNegativeInteger

       Perhaps you should use "@" to indicate the required return type, or
       "$" to specify which version of the function you need.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Where is my mistake?

Thanks

Fabio