https://helageek.blogspot.com/2019/08/c-txt-file-data-update-code.html

#include <iostream>
#include <fstream>
#include <conio.h>
#include <string>

using namespace std;

class User {

public:
	string username;
	string password;

	void userRegister()
	{
		ofstream usersFile ("userData.txt");
		if (	!usersFile.is_open())
		{
			usersFile.open("userData.txt");
		}
	        usersFile << username << " " << password << endl;
	        usersFile.close();
	}

};

int main() {
	
	User user1;
	
	cout << "Welcome!\n-------------------------------\n\nPlease register.\nEnter a new username:\n";
	cin >> user1.username;

	cout << "\nPlease enter a new password:\n";
	cin >> user1.password;

	user1.userRegister();

	getch();
}