In other page you can see to reverse the String if we can make a different copy of the reversed String & suggested way to do this task efficiently.
But what if you want to reverse the original String itself, then we have to edit the original String like it is shown in another article on this site. And how we can use that way to reverse the String is shown below -

import java.lang.reflect.Field;

public class StringReverse {

 public static void main(String[] args) throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {
  String str1 = "Nitin";
  String str2 = "Nitin";
  String sub = str2.substring(3);
  
  System.out.println(str1 == str2);
  
  Field field = String.class.getDeclaredField("value");
  field.setAccessible(true);
    
   char[] chr = (char[])field.get(str1);
  
   int len = chr.length;
   for(int i = 0; i < len/2; i++) {
    char temp = chr[i];
    chr[i] = chr[len-i-1];
    chr[len-i-1] = temp;
   }
  System.out.println(str1);
 }
 
}

Note: If you run above code for Java 9+ then you will get error like-
java.lang.reflect.InaccessibleObjectException: Unable to make field private final byte[] java.lang.String.value accessible: module java.base does not "opens java.lang" to <module_name>
Also, String class is modified a bit, so "value" is not char[] type anymore, to save the space it is byte[] now. So type cast to byte[] & not char[]
Add below as VM arguments while running the code after above type cast change-
 --add-opens=java.base/java.lang=ALL-UNNAMED