I need to edit some stuff. I only knew how to upload but not too sure how to download it though i try to download it using the download option under file manager but no go

You can download it via FTP by double-clicking it or right-clicking it and selected "download" ... essentially the same way you uploaded it to FTP in the first place. With FileManager, just find the file you want, (sessions.php) and click the "download" icon to the right of it. It's usually in the middle of two other icons on either side.