DocBook is an XML-based language. As such, you could edit it with any text editor, but editing large XML documents that way gets very finicky and would also require a pretty thorough understanding of the intricacies of DocBook.

As an alternative, the tool set includes a GUI-based editor, xxe, that "understands" DocBook and can lead you through the options available to you.

To run this editor, give the command

/home/zeil/usr/local/DocBook/tools/xxe-perso-4_5_2/bin/xxe documentName.dbk

The editor is a Java program. With appropriate changes to the path in that command, this should work under Unix, Linux, or Windows.

It's definitely worth putting in some time to go through the "Getting Started" portion of the xxe help and practicing on a few existing DocBook documents.