import java.util.*;
import java.io.*;
public class Test66{
    public static void main(String[] args){
        Properties pro = new Properties();
        try{
            pro.load(new FileInputStream("Test66.Properties"));
        }catch(IOException e){
        }
                
        StringBuffer str = new StringBuffer(pro.getProperty("a")) ;
        str.append(".this is a Test66");
        str.delete(0,4);
        str.insert(0,"this");
        str.replace(40,50,"java");        
        pro.setProperty("a",str.toString());
         
        
        
        try{
            pro.store(new FileOutputStream("Test66_new.Properties"),null);
        }catch(IOException e){
        }
        
    }
}

ps:
a = Here is an example of the contents of a properties file