Editing A Text File

If I have a certain line in a text file, for example "Account Name: Piggy", how do I change the account name so that the line instead becomes "Account Name: Emil". I don't want anything else to change except for the name after "Account Name: ".
This is my code:

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#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
#include <locale>
#include <windows.h>
using namespace std;

void creatingAccount(){
    system("CLS");
    string line, accountname, password;
    fstream inFile;
    inFile.open("exam.txt");
    getline(inFile, line);
    cout << "Enter the desired account name: "; cin >> accountname;
    line = line.substr(1,13) + line.substr(14, accountname.size());
    inFile << line;
    cout << "Enter the desired password: "; cin >> password;
}

void loggingIn(){
    system("CLS");
    string line, line2, defaultaccount, defaultpassword, accountname;
    fstream inFile;
    inFile.open("exam.txt");
    getline(inFile, line);
    getline(inFile, line2);
    defaultaccount = line.substr(14, line.size());
    defaultpassword = line2.substr(10, line2.size());
    cout << "Account: "; cin >> accountname;
    if(accountname != defaultaccount)
        cout << "That account name does not exist!\n" << endl;
    else{
        cout << "Password: "; cin >> accountname;
        if(accountname != defaultpassword)
            cout << "You entered an incorrect password!" << endl;
        else
            cout << "Your money: 5000\n" << endl;
    }
}

int main(){
    char c;
    string line;
    fstream inFile;
    inFile.open("exam.txt");
    if(inFile.fail()){
        cerr << "Error Opening File" << endl;
        exit(1);
    }
    cout << "-= Bank of Shithole =-\n\n";
    while(c != 'E'){
        getline(inFile, line);
        do{
            cout << "What do you want to do?\n(L) Login.\n(C) Create an account.\n(E) Exit.\n"; cin >> c;
            if(islower(c))
                c = toupper(c);
        }while(c != 'L' && c != 'C' && c != 'E');
        switch(c){
            case 'L':
                loggingIn();
                break;
            case 'C':
                creatingAccount();
        }
    }
    cout << "Come again!" << endl;
    inFile.close();
}


This is my text file called "exam.txt":

Account Name: default
Password: default
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