I want to create two communication between parent and child residing in different C files using ordinary pipes (IPC)

I am trying to send message from parant.c to child.c and I am successfully receiving it in the child.c My question is that how can I send message back to the parent using second pipe from child.c I want the exact sequence of code.

Here is my parent.c:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>

int main(int argc, char *argv[])
{
    int fd[2];
    char buf[] = "HELLO WORLD!", receive[100];
    if (pipe(fd))
    {
        perror("pipe");
        return -1;
    }
    switch (fork())
    {
    case -1:
        perror("fork");
        return -1;
    case 0:
        // child
        close(fd[1]);              // close write end
        dup2(fd[0], STDIN_FILENO); // redirect stdin to read end
        close(fd[0]);             // close read end
        execl("./child", NULL); // execute child

    default:
        // parent
        close(fd[0]);                   // close read end
        write(fd[1], buf, sizeof(buf)); // write to write end
        close(fd[1]);                   // close write end
        wait(NULL);
    }

    printf("\nEND~\n");
    return 0;
}

I am sending buf ("Hello world") to the child by executing ./child file. Here is my child.c:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int main()
{
    int fd[2];
    pid_t pid = fork();
    char buf[100], child_msg[] = "From Child: Hello Parent";
    if (pipe(fd))
    {
        perror("pipe");
        return -1;
    }
    switch (pid)
    {
    case -1:
        perror("fork");
        return -1;
    case 0:

        read(STDIN_FILENO, buf, sizeof(buf));
        printf("%s ", buf);
        close(fd[1]);
    default:
        wait(NULL);
        
        
    }
    return 0;
}

I am receiving Hello world in this file. but now how can I send child_msg back to the parent? I don't how to do that. I am stuck at this for last 14 hours.