How Do I Replace A Word In A Text File In C++?

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C++

I am trying to find a word an replace it in the text file. Now, i can find the word successfully in the text file but i am unable to replace it. Can anyone please suggest me where i made the mistake. Thanks.

#include <iostream>
#include <fstream>
#include<string>

using namespace std;

int main(){
ifstream input;
ofstream output;
output.open("Book1.txt",ios::app);

int choice;
size_t pos;
size_t p;
string line;
string a;
string id;
cout<<"enter the name you want to search"<<endl;
cin>>a;

string replace = "hello";
size_t len = a.length();
input.open("Book1.txt",ios::out);
if(input.is_open())
{
while(getline(input,line))
{
pos = line.find(a);
if(pos!=string::npos)
{

line.replace(pos,len,replace);

cout<<"Name://IDnumber//Account-Balance"<<" "<<line<<endl;


}
}
}

system("pause");

}