change wp-admin link or URL in wordpress

would require you to set your installation into a sub-directory, 
for example mydomain.com/mysub-directory/wp-admin 
while mydomain.com/wp-admin will give you a 404 result. 

1- in your public_html (or httpdocs) you should have your wordpress installed on public_html/mydomain/mysub-directory/ 
2- and get the index.php out from public_html/mydomain/mysub-directory/index.php to public_html/mydomain/index.php 
3- edit your index.php and change the wordpress load, which would look like 
/** Loads the WordPress Environment and Template */ 
require( dirname( __FILE__ ) . '/wp-blog-header.php' ); 
to be 
/** Loads the WordPress Environment and Template */ 
require( dirname( __FILE__ ) . '/mysub-directory/wp-blog-header.php' );