Choose a random letter and change it (JAVA)

Hello everyone im new to Java and i need help in my code

    /**
 * @(#)Changing.java
 *
 *
 * @author 
 * @version 1.00 2015/2/27
 */

import java.util.Random;
public class Changing {

    public static void main(String[] args) {
        Random generator = new Random();
        int num1;   
        String phrase = "This is a pencil";

        int len = phrase.length();
        num1 = generator.nextInt(len);
        char c = phrase.charAt(num1, 'x');
        String mut1 = phrase.replace(c, 'x');
        System.out.println(mut1);



    }


}

I need to make progrom to pick a random letter and change it to X

Thank you