Edytowanie pliku tekstowego

Nie wiem jak zedytować plik i go nadpisać. Konkretnie chce napisać metode setMap(x,y), która zamieni np "X" na "Z", gdzie x,y to wspólrzędne tego "X". (y to linia, x to znak w tej lini). Dziekuje za pomoc:) A o to moje pliki:
Map.java

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.io.IOException;

import java.util.*;

public class Map {

private Scanner m;
private static int gridX=19;
private static int gridY=14;
public String Map[] = new String[gridY];

public static int getGridX(){ return gridX; }
public static int getGridY(){ return gridY; }

public Map(){
	openFile();
	readFile();
	setMap(5,5,"P");
	closeFile();
}

public String getMap(int x, int y){
	String index = Map[y].substring(x, x + 1 );
	return index;
}

public void setMap(int x, int y){

	
				
		
			
	}	
		
}
public void openFile(){
	
	
	try {
		m = new Scanner(new File("D:*eclipse*kody*PPPPPacman*src*pacman*Map.txt"));
	} catch (FileNotFoundException e) {
		System.out.println("Nie ma obrazka");
	}
}
public void readFile(){
	while(m.hasNext()){
		for(int i = 0; i <gridY; i++){
			Map[i] = m.next();
		}
	}	
}
	
public void closeFile(){
	m.close();
}
public void changeFile(int x, int y)
{

}

}

Map.txt

XXXXXXXXXXXXXXXXXXX
XYYYXXXYYYYYYYYYYYX
XYYYXYYYYXXXYYYYYYX
XYYYXXXYXXXXYYYYYYX
XYYYYYXYYYYYYYYZZZZ
XYYYYYYYYYYYYYYYYYX
XYXYXYXXXXYYXYYYYYX
XYXYXYYYYXXXXYYYYYX
XYYYXYYYYXYYYYYYYYX
XYXXXXXYYYYYYYYYYYX
XYYYYYYYYYYYYYYYYYX
XYXXXXXYYXYYYYYYXXX
XYYYYYYYYYYYXXXYYYX
XXXXXXXXXXXXXXXXXXX