All you need to edit is in /wp-content/themes/theme-name/
Two ways:
1 – Download files to your machine. make edits. Upload
2 – Go to Presentation > Theme editor then select from what is on the right. This method means that you must make your files ‘writable’ by changing their permissions to 666.
Some info on that:
http://www.tamba2.org.uk/wordpress/ftp/
