Quick question about replacing a character at a specific index in java

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Java

code-challenge

Hello, I have a really quick question about replacing a character in a string at a specific index WITHOUT using nay built-in methods of java.
The task is to be completed with using the for loop only

my input should be: "option number" then phrase "then specific index"
my expected output is " "*" replaced with the character at the specific index"

This is what I tried so far, but I don't know what to put inside the if statement or if my code is wrong


import java.util.Scanner;
public class Phrases{
public static void main(String[]args){
Scanner keyboard = new Scanner(System.in);
int option = Integer.parseInt(keyboard.nextLine());
String phrase = keyboard.nextLine();
if (option == 1){
int x = keyboard.nextInt();
for (int y = 0; y < phrase.length(); y++){
char n = phrase.charAt(y);
if ()
System.out.print(n+"*");
else
System.out.print(n);

What I have tried:

The above code is what I tried