It is very easy to change admin URL for magento. If you want to change URL from

http://www.example.com/admin

TO

http://www.example.com/customadmin

All you need is to properly set your custom admin url in the config file /app/etc/local.xml. In this file change this:

<frontName><![CDATA[admin]]></frontName>

to this:

<frontName><![CDATA[customadmin]]></frontName>

Now save the file and clear the cache (if you are using it). If you still don’t have access to the admin you may clear the cache by deleting the contentss of the /var/cache directory. This is it.