I have Dreamweaver CS6 and want to know if I can edit an existing website built in Front Page if I have the user name and password, and how do I go about it?
You need to Define a site in Dreamweaver and need to set up FTP server. One this is done you can connect with FTP server where your frontpage site is loaded and you can get the copy of that locally.
Link below explains how to do that.
Defining a Site in DW
Connecting to FTP server