How do I insert a forgot password in my code?

#include <stdio.h>
#include <string.h>
#include <windows.h>

void checkGuest();
int calculateDate(int m1, int d1,int y1,int m2,int d2,int y2);

int main()
{
    const char  ADMINUSER[20]="admin",
                ADMINPASS[20]="admin246";

    char user[10],pass[10];
    int access=0;

    system("cls");
    printf("-----------LOGIN INFO-----------\n");

    while (access==0)
    {
        printf("Enter username: \t");
        gets(user);

        printf("Enter pass: \t\t");
        gets(pass);

        if (strcmp(user,ADMINUSER) == 0 && strcmp(pass,ADMINPASS) == 0)
        {
            printf("\n-------Access Granted-------\n");
            access=1;
            Sleep(1000);
            printf("\nAccessing Menu...");
            Sleep(1500);
            checkGuest();

        }
        else
        {
            printf("\n-------Access Denied-------\n");
            Sleep(1000);
        }
    }

    system("cls");
    printf("\n\n\tThank you for using this program!\n\n\n");
    Sleep(5000);

    return 0;
}

//BRINGS UP GUEST INPUT MENU
void checkGuest()
{
    char guest[99];
    int in1, in2, in3;
    int out1, out2, out3;
    int room_num,rate,days,choice=1;
    float amount;

    while (choice!=0)
    {
        system("cls");
        printf("\n-----------Room Rates----------\n");
        printf("\nRoom 1 to Room 5 Single Room (Good For One Person)-\t Php200.00");
        printf("\nRoom 6 to 10 Double Room(Good for two people)-\t\t Php500.00");
        printf("\nRoom 11 to 15 Family Room (Good for 5 people)-\t\t Php900.00");
        printf("\n\n--------Accommodation---------\n");
        printf("\n- Rooms are fully airconditioned\n- Cable Television\n- Comfort Room and Shower with heater\n- Comfortable Beds that accomodates the respective number of people in each room\n- Room Service");

        printf("\n\n------------INPUT------------");
        printf("\n\nEnter Guest Name: ");
        gets(guest);

        printf("Date Of Check-In (MM/DD/YYYY): ");
        scanf("%d/%d/%d",&in1, &in2, &in3);

        printf("Date of Check-Out (MM/DD/YYYY): ");
        scanf("%d/%d/%d",&out1, &out2, &out3);

        rate=0;
        while (rate==0)
        {
            printf("Enter Room Number: ");
            scanf("%d",&room_num);

            if(room_num<=5)
                rate=200;
            else if(room_num<=10)
                rate=500;
            else if (room_num<=15)
                rate=900;
            else
            printf("Room does not exist\n");
        }

        printf("Displaying results...");
        Sleep(1000);

        system("cls");
        printf("\n----------Results----------");
        days=calculateDate(in1,in2,in3,out1,out2,out3);
        amount = days*rate;

        printf("\nGuest: \t\t");
        puts(guest);
        printf("Check-In: \t%d/%d/%d",in1, in2, in3);
        printf("\nCheck-Out: \t%d/%d/%d",out1, out2, out3);
        printf("\nDays staying: \t%d",days);
        printf("\nRoom Number: \t%d",room_num);
        printf("\nTotal: \t\tP%.2f",amount);

        Sleep(2000);
        printf("\n\nCalculate again?\n1-Yes 0-No");
        printf("\n\nChoice: ");
        scanf("%d",&choice);

        fflush(stdin);
    }

    return;
}

//CALCULATES THE DATE
int calculateDate(int m1, int d1,int y1,int m2,int d2,int y2)
{
    int days=0;
    int month,months[12]={31,28,31,30,31,30,31,31,30,31,30,31};

    if(y1==y2 && m1==m2)
    {
        days=d2-d1;
    }
    else
    {
        if (m2>m1)
        {
            month=m1;

            while (month<=m2)
            {
                days=months[month-1]+days;
                month++;
            }

            days=(days-(d1+(months[m2-1]-d2)));
        }
        else if (m1>m2)
        {
            month=m2;

            while (month<=m1)
            {
                days=months[month-1]+days;
                month++;
            }

            days=days-((months[m1-1]-d1)+d2);
        }

        if (y2 > y1)
        {
            int year=0;

            year=365*(y2-y1);

            if (m2>m1)
                days = year + days;
            else if (m2<m1)
 days="year" -="" days;
="" else="" if="" (m2="=m1" &&="" d1="">d2)
                days = year - (d1-d2);
            else if (m2==m1 && d2>d1)
                days = year + (d2-d1);
        }
    }
    return days;
}

What I have tried:

//Im kind of having a hard time changing the password into a changeable one as our prof asked us to put a forgot password feature. its in c programming