Download the fresh copy of that file first, go to theme download page and download the theme, once unzipped look inside the folder you will see only one file named
functions.php this is it.
Once connected to your server via FTP, locate
wp-config.php, when you see that file look for a folder called
wp-content go inside this folder, then go inside
themes and then
WARNING: If you are not comfortable working with files live on the server, don't do it yourself because you will risk making more serious mistakes, let the person who installed WP for you take care of this.