If you still have a copy of the full website in its root folder then all you do is make the amendments to the page(s) you want to adjust then when you publish the site choose the option to publish changed pages only. This won't overwrite anything on the site other than pages that have been altered. The web server files are timestamped and FrontPage will compare the date/time/size of the files and only upload those files that have been edited and altered since the last publish.
If you've lost your copy of the site on your PC, start up FrontPage, create a new site and select the Import Web Site Wizard. Follow the prompts and it will import a full copy of the site to your PC for you to work on. It will tak a while, mainly becasue some of the files on your site a pretty huge.