Create a folder on your hard disc.
Use FTP to download your website and store it in this folder.
IMPORTANT - Make a backup copy of the folder and contents.
Open FrontPage, then use File->Open Site and browse to the folder. Allow
FP to convert the folder to a web.
Edit the site as best you can. It may be easier to start over since
sitebuilders often use really bad code.
When ready, publish the site back to Yahoo, either using File->Publish
Site, or using your FTP application.
Alternatively, install the FP extensions on your Yahoo site, and publish
back with FrontPage in HTTP mode - though as far as FrontPage is concerned,
Yahoo do not appear to be very good hosts.