Suppose current admin url is http://www.yoursite.com/admin and you want to change it http://www.yoursite.com/administrator.
Step 1. Navigate to the app/etc/local.xml and open the file in the editor of your choice.
Step 2. Search for the code below
<frontname><![CDATA[admin]]></frontName>
Step 3. Change the admin to administrator or whatever you want, then the code will look like as:
<frontname><![CDATA[administrator]]></frontName>
Step 4. Now Remove the cache Manually, for the Manual cache removal, Navigate to the var/cache/ select all the items and remove it.
Step 5. Now try to open your admin with the new url http://www.yoursite.com/administrator.
Hope this will be helpfull.
Thanks, and Happy coding.