There is a difference between decompiling and editing a jar content.
A jar (java archive) as its name says is an archive or collection of java classes that are ready to be executed.
A java decompiler is a tool that makes a reverse engineering from .class extension to .java extension.
I do not know what is your need, anyway check how we solve both issues.
For editing the content of the jar file (adding, removing, decompiling) you can go to the command prompt (or shell terminal for linux or unix based os) and on which you have installed the JDK not the JRE and you can write the following command:
and it will scroll down in front of you all the possible combinations of options on how to use jar (for example to explode the content of a jar you can write: jar -xvf (full path to jar file)
On the other hand if you want to decompile a class file into a java file, you can download the jad (java decompiler) from the following url: