Updating .class file in jar

This tutorial details how to update a jar file

jar -uf jar-file <optional_folder_structure>/input-file(s)     

where 'u' means update.

Do you want to do it automatically or manually? If manually, a JAR file is really just a ZIP file, so you should be able to open it with any ZIP reader. (You may need to change the extension first.) If you want to update the JAR file automatically via Eclipse, you may want to look into Ant support in Eclipse and look at the zip task.