Open the site/web (File > open web/site) in Frontpage, then open a page,
then make the changes and re-publish them. In the same way that you started
doing the site. Open the page(s) to modify them.
The above assumes you're working on the local machine and publishing;
otherwise, open the live site through FP (File > open web/site) and put in
the full URL to the site, enter the user logon details, then once connected
open the pages you want to change.
This assumes you have the FP server extensions installed on the server; ask
your host if this is not the case.
If the FPSE are not installed, then publish using an FTP program, or the FTP
mode in Frontpage.