A jar file is a zip archive. You can extract it using 7zip (a great simple tool to open archives). You can also change its extension to zip and use whatever to unzip the file.
Now you have your class file. There is no easy way to edit class file, because class files are binaries (you won't find source code in there. maybe some strings, but not java code). To edit your class file you can use a tool like classeditor.
You have all the strings your class is using hard-coded in the class file. So if the only thing you would like to change is some strings you can do it without using classeditor.
Here's what I did:
That's it. I have tested it with my Nokia and it's working for me.
This is a tool to open Java class file binaries, view their internal structure, modify portions of it if required and save the class file back. It also generates readable reports similar to the javap utility. Easy to use Java Swing GUI. The user interface tries to display as much detail as possible and tries to present a structure as close as the actual Java class file structure. At the same time ease of use and class file consistency while doing modifications is also stressed. For example, when a method is deleted, the associated constant pool entry will also be deleted if it is no longer referenced. In built verifier checks changes before saving the file. This tool has been used by people learning Java class file internals. This tool has also been used to do quick modifications in class files when the source code is not available." this is a quote from the website.
Source: Stackoverflow.com