You can use iThemes Security to change WordPress Admin Login URL.
Step 1. Install iThemes Security (formerly Better WP Security)
Step 2. Activate
Step 3. Security > Settings
Step 4. Advanced > Configure Settings in Hide Backend
Step 5. Tick on Enable the hide backend features
Step 6. Change WordPress Admin Login URL
Done!
To reset WordPress Admin Login URL to default, simply deactivate plugin.
If you can not access the website, rename the iThemes Security folder (or Better WP Security folder) on Your Hosting.