modifying a class file inside jar

I want to modify a .class file inside a jar file and put it back ..
How to do this?

 

A class file contains bytecode, you'll to decompile it to the source and the modify and recompile!

 

I decompiled into source code(I mean the java files)

Now I cannot compile it seperately...

Actually I tried like this in the cmd prompt.... I added required jar files in the classpath and compiled.. There is a statement like this "ClassNotFoundException x1" which gives an error - invalid declaration