How to swap matrix quarters clockwise

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C++

algorithm

Matrix

I want to write a program that:
1) Generates quadratic matrix of size (2Nx2N)
2) Prints this matrix
3) Swaps the quarters of the matrix clockwise (beginning from top left corner)
4) Prints changed matrix.

For example, the output of my program should be like this:

01 02 03 04 
05 06 07 08 
09 10 11 12
13 14 15 16

Matrix (changed)

09 10 01 02
13 14 05 06
11 12 03 03 
15 16 07 08

Here is my source code:

#include <iostream>
#include <stdlib.h>
#include <time.h>
#include <iomanip>
using namespace std;

const int n = 4; // size of matrix

void CreateMatrix(int matrix[][n], int n); // functions' prototypes
void PrintMatrix(int matrix[][n], int n);
void ProcessMatrix(int matrix[][n], int n);
int main()
{
    int matrix[n][n];

    srand(time(NULL));

    CreateMatrix(matrix, n);    //create matrix
    PrintMatrix(matrix, n);     //print matrix
    ProcessMatrix(matrix, n);   //change matrix

    cout<<"\nMatrix(changed):\n"; 

    PrintMatrix(matrix, n);    // print changed matrix

}
void CreateMatrix(int matrix[][n],int n)
{
    for (int i =0; i<n; i++)
        for(int j = 0; j<n; j++)
            matrix[i][j] = rand()%10;
}
void PrintMatrix(int matrix[][n], int n)
{
    for(int i=0; i<n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            cout<<matrix[i][j]<<" ";
        }
        cout<<"\n";
    }
}
void ProcessMatrix(int matrix[][n], int n) 
{
    // I don't know how to write this function
}

So my question is how to write ProcessMatrix function?
Any help is appreciated!