Create a Matrix:

cv::Mat Matrix= (cv::Mat_<float>(3, 3) << 0, 1, 0, 1, -4, 1, 0, 1, 0);

Use the Mat_ Object to access the values:

cv::Mat_<float> Matrixvals= Matrix;

Matrixvals(1,0) = 2;

Matrixvals(1,1) = -8;

Matrixvals(1,2) = 2;