#21

            Random random = new Random();
            int maxNumber = int.MinValue;
            int setNumber = 0;
            int maxRandomNumber = 5;
            int minRandomNumber = 1;
            int matrixLineIndex = 10;
            int matrixColumnIndex = 10;
            int[,] matrix = new int[matrixLineIndex, matrixColumnIndex];

            for (int i = 0; i < matrix.GetLength(0); i++)
            {
                for (int j = 0; j < matrix.GetLength(1); j++)
                {
                    matrix[i, j] = random.Next(minRandomNumber, maxRandomNumber + 1);

                    if (maxNumber < matrix[i, j])
                    {
                        maxNumber = matrix[i, j];
                    }

                    Console.Write(matrix[i, j] + " ");
                }

                Console.WriteLine();
            }

            Console.WriteLine();

            for (int i = 0; i < matrix.GetLength(0); i++)
            {
                for (int j = 0; j < matrix.GetLength(1); j++)
                {
                    if (maxNumber == matrix[i, j])
                    {
                        matrix[i, j] = setNumber;
                    }

                    Console.Write(matrix[i, j] + " ");
                }

                Console.WriteLine();
            }

            Console.WriteLine($"\nНаибольший элемент матрицы: {maxNumber} \n");