see the changes below

Code:
```//This program prints the initialized value of two complex numbers and allows
//the user to change the values of the real and imaginary numbers of the two
//complex numbers. This program can also solve the sum and the product of the
//two complex numbers and display all the complex numbers in the program.

#include<stdio.h>
#include <conio.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>

typedef struct CN{
double cxr,cyr,czr;
double cxi,cyi,czi;
}CN;

CN * Sum(CN *CN1){
CN1->czr=CN1->cxr+CN1->cyr;
CN1->czi=CN1->cxi+CN1->cyi;
return CN1;
}
CN * Mul(CN *CN1){
double a,b,c,d;
a=CN1->cxr*CN1->cyr;
b=CN1->cxr*CN1->cyi;
c=CN1->cxi*CN1->cyr;
d=-1.0*(CN1->cxi*CN1->cyi);
CN1->czr=a+d;
CN1->czi=b+c;
return CN1;
}

void DisplayCN(CN *p);

int main(void){
CN *ptr;
char error[128];
int selection;
ptr=(CN *) malloc(sizeof(CN));
ptr->cxr=1.0;//first number
ptr->cxi=-2.0;

ptr->cyr=3.0;//second number
ptr->cyi=4.0;

do{
system("cls");
printf("The Starting Values:\n");
printf("CN1: %.2f%+.2fi\n",ptr->cxr,ptr->cxi);
printf("CN2: %.2f%+.2fi\n\n",ptr->cyr,ptr->cyi);
printf("Complex Number Selection Screen\n\n");
printf("First Complex Number:\n");
printf("1 - Change the Value of the Real Number\n");
printf("2 - Change the Value of the Imaginary Number\n\n");
printf("Second Complex Number:\n");
printf("3 - Change the Value of the Real Number\n");
printf("4 - Change the Value of the Imaginary Number\n\n");
printf("Complex Number Operations:\n");
printf("5 - Add the two Complex Numbers\n");
printf("6 - Multiply the two Complex Numbers\n");
printf("7 - Display all the Complex Numbers\n");
printf("8 - Exit\n");
fgets(error,128,stdin);
selection=atoi(error);
switch(selection){
case 1:
system("cls");
printf("Change the Value of the Real Number\n");
printf("CN1: %.2f%+.2fi\n",ptr->cxr,ptr->cxi);
printf("New Value for the Real Number: ");
scanf("%lf",&ptr->cxr);//CN1.cxr);
getchar();
getche();
break;
case 2:
system("cls");
printf("Change the Value of the Imaginary Number\n");
printf("CN1: %.2f%+.2fi\n",ptr->cxr,ptr->cxi);
printf("New Value for the Imaginary Number: ");
scanf("%lf",&ptr->cxi);//CN1.cxi);
getchar();
getche();
break;
case 3:
system("cls");
printf("Change the Value of the Real Number\n");
printf("CN2: %.2f%+.2fi\n",ptr->cyr,ptr->cyi);
printf("New Value for the Real Number: ");
scanf("%lf",&ptr->cyr);//CN1.cyr);
getchar();
getche();
break;
case 4:
system("cls");
printf("Change the Value of the Imaginary Number\n");
printf("CN2: %.2f%+.2fi\n",ptr->cyr,ptr->cyi);
printf("New Value for the Imaginary Number: ");
scanf("%lf",&ptr->cyi);//CN1.cyi);
getchar();
getche();
break;
case 5:
system("cls");
printf("Sum of the Complex Numbers\n");
ptr=Sum(ptr);
printf("The Sum of the Complex Numbers is %.2f%+.2fi\n",ptr->czr,ptr->czi);
getche();
break;
case 6:
system("cls");
printf("Product of the Complex Numbers\n");
ptr=Mul(ptr);
printf("The Product of the Complex Numbers is %.2f%+.2fi\n",ptr->czr,ptr->czi);
getche();
break;
case 7:
system("cls");
DisplayCN(ptr);
break;
case 8:
exit(1);
break;
default:
system("cls");
printf("Please press the spacebar to continue!!");
getche();
break;
}
}while(selection!=8);
}

void DisplayCN(CN *p){
printf("The Values of the Complex Numbers:\n");
printf("CN1: %.2f%+.2fi\n",p->cxr,p->cxi);
printf("CN2: %.2f%+.2fi\n\n",p->cyr,p->cyi);
printf("Sum of the Complex Numbers\n");
p=Sum(p);
printf("%.2f%+.2fi\n\n",p->czr,p->czi);
printf("Product of the Complex Numbers\n");
p=Mul(p);
printf("%.2f%+.2fi\n",p->czr,p->czi);
getche();
}```