General Editing the PSD?

Nope not at all. All you are doing is changing the psd file which is all to it's self.
Think of it this way - you create a graphic of what you want your website or page to look like (one large graphic), you then cut out the parts you may need when you code the page.
You can see the parts in the images file. If you look at the code in both the style.css and the index.html file you will see references to those different parts.
Also you will see different specs as to how to display those parts on the page so the browser can interpret the page and display it to your viewers as you intended.

You can change the individual images, but keep the same name or you will have to alter the code in either or the css/html files.
If you understand the process of slicing/dicing a psd file then you can alter the psd, though that is more of starting from scratch method.

In some cases the graphic you are altering has several layers to it and you only want to remove or alter one of them, then it is easier to alter the psd file for that one part, but you will have to save just that part.

You may want to change a background color - which may just mean changing to color code in the css file.

As you can see there are different methods to achieve what you need.
Hope this helps.