Joomla can be a pain with this sometimes.

The first thing to try is to go into your FTP program and see if you can set the permissions for that file to 777.

If that fails, then you will need to do the following:

Go to http://extensions.joomla.org and search for eXTplorer

download this and install it on your site. then, within this, go find the offending file and change its permissions to 777.

Why do you need to do this?

In PHP each file has an "owner". When you upload files by FTP, then the owner of that file is you - by which I mean your FTP login username.

However, when the file is created by a script within your server, such as Joomla, then the file is owned by a user called "nobody".

Unfortunately, if a file is owned by "nobody", and the permissions are set to not allow people to write to that file, then you (ie your FTP username) does not have permission to change or delete that file, even by loging in through FTP or cPanel. That's right, you don't have permission to change your own files!

Which is why Joomla built the eXTplorer so that you can have access to nobody's files. It is a pain in the neck, but you need only do it once.

For more information on this, see this excellent article:

http://support.inmotionhosting.com/cgi-bin/kb.cgi?do=read&id=45&lang=en