This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff 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.