How to edit the source code of a leaked plugin?

[MikeyJY]

I want to edit the source code for MC Phones: https://blackspigot.com/downloads/mc-phones.4121/ , but in the
decompiled folder I found:

1. a.java
2. b.java
3. c.java
4. d.java
5. e.java
6. f.java
7. g.java
8. H.java
9. i.java
10. k.java
11. l.java

But all thees files contains a bytecode written as a comment!