You have to edit the Forum-Adresse in the DB settings.

thnx, where exactly?
in a file or logged in as administrator in the forum?

The former. Edit db_settings.php reflecting you host.

$db_settings['host'] = "your host";
$db_settings['user'] = "your user name";
$db_settings['pw'] = "your password";
$db_settings['db'] = "your name of the data base";

Upload by FTP and don’t forget to set permissions to 664.