1) Editing is fine. However, edit for fun/practice, do not edit for publish/use. I actually enjoy editing stuff, but it's usually to help someone out. So ya, edit to your hearts content, but leave it at that.

2) Realize the amount of impact a single pixel has at these resolutions. Imagine you're making a head, and you have a 2x3 block of white pixels for the eye. A single pixel on the left or right side of that completely changes the context of your head and can easily make your head look very cross-eyed. Move it over a pixel and bam, it looks fine. These are things you need to remember as you constantly move around and tweak the pixels. A single pixel can make or break your head, so don't be afraid to constantly experiment and change things.

3) Most importantly, at the core of all art, including pixel art, should be a solid knowledge of fundamentals. Pixel art is a very unique form of art and it does take specific knowledge of various techniques(like number 2), but in the end strengthening your fundamentals will make you a better pixel artist. This is always applicable. Knowledge of lighting, volume, forms, color theory... it's all universal. So if you truly want to get better as a pixel artist, get better as an artist.  

Thanks Dusty.That helped a lot.I have permission from the maker of the head I edited to use the new copy I made,but I prefer I'll experiment by making a few heads of my own first so i might use one of them instead.