Its very simple, you cant.
There is no option to change the administrator url
Here is why


Joomla as well as quite a few 3rd party extension rely on the "administrator" folder. If you change the folder name it will break your site.

Instead you could password protect your administrator directory. If you have cPanel this is easy. That way if someone goes to yoursite.com/administrator all they will see is a popup login box asking you for a username and password and a blank white page behind it.

Another option is to use .htaccess to password protect or even only allow access from a specific IP.

There are also extensions such as jSecure which requires you to have an access key to be able to access the login page.

http://extensions.joomla.org/extensions/access-a-security/site-security/login-protection

If your host uses cPanel or provides a way to password protect, I would go that route as it is the easiest and you then wouldn't rely on a 3rd party extension or have to add code to .htaccess

Mark

There are Extensions that redirect the  http://www.site.com/administrator url to  http://www.site.com. And require a url like  http://www.site.com/administrator?passcode to reach the Admin login screen.

Kevin