Customer data file code in C.Facing issues - c

Question

The code is supposed to Accept Data,Display the entire data stored till the current execution,Modify or make changes to the already accepted data.
WHAT IM LOOKING FOR IN AN ANSWER:how to write the code for a Modify module?
how to correct the Add or accept data,Display
data so they function perfectly.
#include<stdio.h>
typedef struct
{
int select;
char lastname[25];
char firstname[25];
char address[25];
int phonenumber[25];
} addressbook;
#define ARRAYLEN 2 //The lecturer told to use this "#define ARRAYLEN 2
addressbook a[ARRAYLEN]; //but why 2?and why use ARRAYLEN 2 with define
FILE *fp;
int main() //how is it affecting my program
{
int i;
int c;
int fgetc(FILE *stream);
int fputc(int c,FILE *stream);
fp = fopen("addressbook.dat","a+");
//ADD
for( i=0; i<ARRAYLEN ; ++i)
{
printf("enter details\n");
printf("enter lastname:\n");
scanf("%s", a[i].lastname);
printf("enter firstname:\n");
scanf("%s", a[i].firstname);
printf("enter address:\n");
scanf("%s", a[i].address);
printf("enter phone number:\n");
scanf("%d", a[i].phonenumber);
fwrite(&a[i], sizeof(a), 1, fp); /* notice, array indexed */
};
fclose(fp);
//DISPLAY
fopen("addressbook.dat", "r");
for(i=0; i<ARRAYLEN; ++i)
{
fread(&a[i], sizeof(a), 1, fp );
fgetc;
printf("first name is %s",a[i].firstname);
printf("last name is %s",a[i].lastname);
printf("the address is %s",a[i].address);
printf("the phonenumber is %d",a[i].phonenumber);
};
fclose(fp);
return 0;
}
IMPORTANT NOTE: I use a windows vista os and the compiler is Turbo C++