I cannot Edit sidebar.php

I have downloaded a bunch of nice Primium WPThemes and I have been trying with no success to edid the sidebar.php
I had no problem with
# Main Index Template
# Header
# Footer
and placing my ID for Goole ads

I need to replace the one on the sidebar. However, I have no idea how to. Each time I replace the ID number with mine and update the file I get the following message:
"Forbidden
You don't have permission to access /wp-admin/theme-editor.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request".

Anyone that would not mine to help?:

You may need to change permission of the file to 777. Better download the file using FTP, edit it and upload.