Title changed to avoid trolling

It's fairly close to C already, apart from the I/O. No one here will do it for you; you need to research it yourself. But I'll give you a hint. Remove

#include <iostream>
using namespace std;

and replace it with

#include <stdio.h>

Then find some documentation for stdio.h and decide how to fix the things that no longer compile now that iostream is gone.

Here is the converted code, using printf and scanf to replace C++'s cout and cin respectively.

#include <stdio.h>

#define N 10

int main()
{
    int array[N], p, q;
    for (int i = 0; i < N; i++)
        array[i] = i;

    printf("For instance, given elements are ");
    for (int i = 0; i < N; i++)
        printf("%d ", array[i]);
    printf("\n");
    int r = 0;
    scanf("%d", &p);
    while (p>0)
    {
        scanf("%d", &q);
        if (p >= 0 && p < N)
        {
            for (int i = 0; i < N; i++)
                if (array[i] == p)
                    array[i] = q;


            printf("\n%d %d\n", p, q);
            for (int i = 0; i < N; i++)
                printf("%d ", array[i]);
            printf("\n");
        }
    }
    return 0;
}