FixTag is a Java/Swing application that lets you edit the fields within MP3 ID3 tags.
FixTag is written in Java and requires a Java runtime version 5.0 or newer to run. You can download Java from Sun Microsystems here.
If you have a Java runtime installed on a Windows system, you should be able to simply double-click the fixtag jar file to run the program.
If that doesn't work, or on a UNIX system, you need to ensure that the Java runtime is in your PATH by typing
at a command prompt. If Java spits back a version string, then you can type
java -jar fixtag-x.xx.jar
to get it running (where fixtag-x.xx.jar is the name of the jar file you downloaded from SourceForge).