I do not use iWeb but usually there are two places where your website exists, online on the hard drive of your web host (which is the version anyone can access via the web), and often a duplicate on your own computer. When you update or change any web pages on your home computer you can then upload that on to your host provider via any number of software utilizing File Transfer Protocol (ftp). Anyone can access your host provider from any computer as long as they know the access address and password. So now-a-days you can update your website from any of your computers, your iPad, or your smart phone. For example, if you were visiting my house, or at a net-cafe on the road, you could easily update your website, as long as you had the password necessary to access your host provider.
Hope this was clear and helpful.
You can also download copies of your various web pages via FTP from your web host to your laptop or another computer. That is what I do when I want to assure that I have the most current ones from which to make changes.
They then will typically open in a web browser such as Safari; however, to make changes, which I still do by hand, I open them in Tex-Edit Plus, my favorite text editor, to show the html code.
On my computer(s) the pages reside in a Web Site folder. Usually I change the titles from, say, index.html to index2-21-11.html, to keep a record of their order. Before I upload one, I remove the date since that would break the links and thereby confuse my web host server.
Although I do not use iWeb, or even RapidWeaver yet, I assume that you could open it and then open the pages from within it -- as I do with a text editor.
Norm