editing a ASP . NET site

Hi,
If I have a website application built in ASP.NET do I need visual studio to edit it?
Do I install visual studio and then connect to the FTP host and away I go?

I want to modify a few simple things but I cant find or read the code by the simple download and open in notepad method. I assume all the code I need to get at is within the dll files.

Cheers
Mitch