To change admin login URL you need to:
1. Open
conf.php file (conf.php is the core configuration file. You can find it inside core root folder). Search for
j_admin_folder and change the value to what you want:
define('j_admin_folder','new-value');
2. Rename admin folder in core root folder.
Now, the Administrator back-end can be reached by appending the word that you specified in j_admin_folder to the URL of your site. For example http://example.com/new-value/.