Extending Width – How Do I Re-Center? Please help

Hi

I’m trying to extend the width of the main content area. I’ve located all of the files, graphics etc I need to change in width. However when I add in the new width…it just extends out to the right of the page.

The site is centered by default – but when I change the width it extends out to the right. I need it to become wider but ‘remain’ centered.

How do I make sure the site re-centers?

Thanks in advance
Ian