yousite/sites/default and open the
settings.php file and find
$base_url and change it to the new value (somewhere around line 270).
settings.php is a read-only file, you need to clear the read-only flag first to make the changes. You'll need to restart the server to update the changes.
There is a global variable called
$base_url that you can set in sites/default/settings.php. Try changing its value.