help with login system

[xxgixxx]

hi, i made a login system where i can change my info.
but when i change the info i can 't use the new ones when i run again.
i know that original strings are back to initial username and password.
i want to know how to put them in a .txt file and delete them then make new ones to get the info i edited in the second run of the program.
i suggest you to run the program so you can understand the code.
here is the code:

///login system
///gilbert Sassine

#include <cstdlib>
#include <iostream>
#include <windows.h>

using namespace std;
void seconds_count();

const int MAX_WRONG = 5;
string username = "gilbert";
string password = "toronto77889900";
int main()
{
    system ("color 1f");

    MAINLOOP:

    int wrong = 0;
    string userChoice;
    string userChoice2;

SYSTEM_LOOP:

    system ("cls");
    cout << "Username : " << endl;
    cin >> userChoice;
    cout<<endl;

    cout << "Password : " << endl;
    cin >> userChoice2;
    cout<<endl;

    if (userChoice == username && userChoice2 == password)
    {
        cout<<"correct !"<<endl;
        system ("pause");
        system ("cls");
    }
    else if (userChoice != username && userChoice2 != password)
    {
        cout<<"incorrect !"<<endl;
        wrong++;
        system ("pause");

        if (wrong == MAX_WRONG)
        {
            seconds_count();
        }
        goto SYSTEM_LOOP;
    }
    else if (userChoice == username && userChoice2 != password)
    {
        cout<<"incorrect !"<<endl<<endl;
        system ("pause");
        goto SYSTEM_LOOP;
    }
    else if (userChoice != username && userChoice2 == password)
    {
        cout<<"incorrect !"<<endl<<endl;
        system ("pause");
        goto SYSTEM_LOOP;
    }

cout<<"1) enter game"<<endl;
cout<<"2) edit username or password"<<endl;
cout<<"3) Exit "<<endl;

int choice;
cin>>choice;

switch (choice)
{
case 1:
    cout<<"you are in the game"<<endl;
    break;

case 2:
    enter_username:

    system ("cls");
    cout<<"username : "<<username<<endl;
    cout<<"password : "<<password<<endl;


cout<<"enter new username :"<<endl;
username = "";
cin>>username;
cout<<endl<<endl;

cout<<"enter new password"<<endl;
password = "";
cin>>password;
cout<<endl<<endl;

cout<<"re-enter your new password"<<endl;
string password_confirm = "";
cin>>password_confirm;
cout<<endl<<endl;

if (password_confirm != password)
{
    cout<<"passwords didn 't match"<<endl;
    username = "";
    password = "";
    system ("pause");
    goto enter_username;
}
else if (password_confirm == password)
{
    cout<<"username and password changed successfully"<<endl;
    cout<<"please login to continue..."<<endl<<endl;
    system ("pause");
    goto MAINLOOP;
}
break;
if (choice == 3)
{
    goto END;
}
}
END:

return 0;
}
void seconds_count()
{
    for (int c = 30 ; c > 0 ; c--)
    {
        cout<<"Please wait "<<c<<" seconds"<<endl;
        Sleep (1000);
        system ("cls");
    }
}

thank you !
return 0;