Site broken after using URL Change in Admin

Best way

If you have access to the site via FTP, then this method will help you quickly get a site back up and running, if you changed those values incorrectly.

1. FTP to the site, and get a copy of the active theme’s functions.php file. You’re going to edit it in a simple text editor and upload it back to the site.

2. Add these two lines to the file, immediately after the initial <?php line:

update_option('siteurl', 'http://example.com');
update_option('home', 'http://example.com');

Use your own URL instead of example.com, obviously.

3. Upload the file back to your site, in the same location. FileZilla offers a handy “edit file” function to do all of the above rapidly; if you can use that, do so.

4. Load the login or admin page a couple of times. The site should come back up.
   Important! Do not leave those lines in the functions.php file. Remove them after the site is up and running again.

Note: If your theme doesn’t have a functions.php file create a new one with a text editor. Add the php tags and the two lines using your own URL instead of example.com:.

<?php
update_option('siteurl','http://example.com');
update_option('home','http://example.com');

Upload that to your theme directory. Remove the lines or the remove the file after the site is up and running again.

Source: WordPress.org – Changing The Site URL