Editing sharpfolio theme style sheets

Hi, I need to edit the style sheet in my Sharpfolio Template but the following message prevents me saving any changes – ‘you need to make this file writable before you can save your changes. See the Codex for more information.’
I have looked at threads and checked this link,
http://codex.wordpress.org/Changing_File_Permissions
but to be honest I’m not at all clued up on this and struggling to understand it. Is there not an easy way to explain how to change the permissions?
As always, any advice greatly appreciated
Thanks
A