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.
