1. Add these two lines to the file, immediately after the initial "<?php" line
update_option('siteurl','http://worpresssite.com');
update_option('home','http://wordpresssite.com');
2. Load the login or admin page. The site should update to the new URL in step 1.
3. IMPORTANT: Remove the lines you added. Do NOT leave those lines in there. Remove them immediately after the site is up and running again