[virxen]

as a start do the following

........
void DisplayCN(CN,CN,CN);//add this

int main(void){
.........

insert a getchar(); after the scanf's before getche();

for example

..............
      case 1:
        system("cls");
        printf("Change the Value of the Real Number\n");
        printf("CN1: %.2f%+.2fi\n",cx.real,cx.img);
        printf("New Value for the Real Number: ");
        scanf("%lf",&cx.real);
        getchar();
        getche();
        break;
      case 2:
.............