How to change already compiled .class file without decompile?

You can follow these steps to modify your java class:

1. Decompile the .class file as you have done and save it as .java
2. Create a project in Eclipse with that java file, the original JAR
   as library, and all its dependencies
3. Change the .java and compile
4. Get the modified .class file and put it again inside the original
   JAR.