You mean to edit the PHP code? Use a plain text editor like Notepad (don't use a rich text editor such as Word - that will do bad things to the code). If you want extra features like syntax highlighting, code completion and debugging then consider getting a program like Dreamweaver, Visual Studio with VS.Php or Zend Studio (though none of these are free). There's a big list of free and commercial PHP editors
here you could look through too.
Edit: One problem you may have with Notepad is all of the text appearing on one line. This happens if the file has been saved with Unix line endings. Choosing a more advanced editor (including many of the free ones) should solve this.