If you just want to edit text, notepad will do you fine, as you can open up the code, edit what you like and rebuild. But if you were planning to add/remove things it would definitely be worth downloading a proper IDE, like Visual Studio, which you have.
After you've edited whatever you like in Visual Studio, just press F6 and it'll re-compile the whole thing for you, and overwrite the EXE in bin/release
Open the solution (.sln) in Visual Studio, and Form1.cs will be on the right. Right click on it, and view code. That'll bring up the code for Form1. Then edit away, and press F6 to save and rebuild