if we could take a look at the site Steve, could you post the url (address)?
Editing a site is pretty easy, once you have it loaded into a WYSIWYG (What You See Is What You Get) HTML editor like FrontPage or NetObjects Fusion.....at least that's the theory. In practice things don't always go to plan, but you can protect the situation by running a mirror of the site's directories and files on your hard drive. This is the version that you'll work with, and you won't publish your changes to the server where your 'real' site is running until you're happy with the results.
If you want to be hyper-cautious (and there's nothing wrong with being like that in computing)you'll maintain an archive copy as well, so that you can always revert to it if you publish to the server in error - leaving you with a faulty working copy as well as a faulty live copy.
That's the dire warning over with - all you need to do now is get some software. It doesn't particularly matter what you use, as long as it's reasonably well specified. The caveat is that I haven't seen your site, and I'm assuming it's fairly conventional - no databases or Flash animations.
Let us have that site address, and we'll take this to the point where you can get your hands on your site to update it.