I am locked out of my WordPress Dashboard while changing the Site URL


I have been preparing my site in a Subdirectory and now wanted to take the site live by allowing access from the top-level domain. To do this I updated the Site URL and Web Address in the Dashboard settings and followed the instructions given on the WordPress documentation for Using a pre-existing subdirectory install. (http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory) This involved copying index.php and .htaccess files from the subdirectory to the home directory. I made the required change in the index.php to point at WordPress in the subdirectory.

Now my site will not load properly and if I attempt to login to my Dashboard via wp-login.php I get an error message saying “too many redirects”

How can I restore my WordPress Dashboard and site to a working state?