This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Do you mean you want to replace the .class file? If your application is already deployed and you want to replace the .class file, you can do that but the changes will not be reflected unless you redeploy the application.