Introduction

I have used CodeBlocks compiler for debugging purpose. But you can use any C++ programming language compiler as per your availability.

#include <iostream>
using namespace std;
 
int main ()
{
    static int array[10][10];
    int i, j, m, n, a;
 
    cout<<"Enter the order of the matix \n";
    cin>>m;
    cin>>n;
    if (m == n)
    {
        cout<<"Enter the co-efficients of the matrix\n";
        for (i = 0; i < m; ++i)
        {
            for (j = 0; j < n; ++j)
            {
                cin>>array[i][j];
            }
        }
        cout<<"The given matrix is \n";
        for (i = 0; i < m; ++i)
        {
            for (j = 0; j < n; ++j)
            {
                cout<<" "<<array[i][j];
            }
            cout<<"\n";
        }
        for (i = 0; i < m; ++i)
        {
            a = array[i][i];
            array[i][i] = array[i][m - i - 1];
            array[i][m - i - 1] = a;
        }
        cout<<"The matrix after changing the \n";
        cout<<"main diagonal & secondary diagonal\n";
        for (i = 0; i < m; ++i)
        {
            for (j = 0; j < n; ++j)
            {
                cout<<" "<<array[i][j];
            }
            cout<<"\n";
        }
    }
    else
        cout<<"The given order is not square matrix\n";
}

Result