Oh dear.
Well you have to download the .php file first.
Open Smartftp, connect to your FTP server.
Go to your Vb Dir, select the file you want, and drag and drop it to your desktop, or you can right click ---->Download, and the specify a directory

Then open the files in Notepad from where ever you download the file to. And then it's pretty straight forward to edit the file, just follow the instructions to the point.
Once you've edited it, open up smart ftp, this time drag and drop the newly saved file (the one you just edited) to your FTP server, overwriteing the older .php file.