In LyX you are supposed to change the file in the main window, not in the ...
In LyX you are supposed to change the file in the main window, not in the "LaTeX Source" frame (this is just informative, and you can watch --- via "Update" button --- how source file reflects the changes you make in the main window).
Any text editor can be used for editing TeX and LaTeX source files; Geany and Emacs have good templates for this.
But to do this, you must learn something about LaTeX. I first understood what is it and how it works from the excellent book by Peter Flynn: Formatting information. A beginner's introduction to typesetting with LATEX (http://www.tex.ac.uk/tex-archive/info/beginlatex/html/).