If you forgot the password of the administrator on On-premises installation, please open config.inc.php and edit force_reset_accounts to reset the accouts' password.
You still need to remember the account's email, if you do not modify it, the default should be: firstname.lastname@example.org
$config['force_reset_accounts'] = array(
array('email'=> 'email@example.com', 'passowrd'=> 'my new password')
This will reset the password to "my new password" for the account.
You need to modify this entry to
$config['force_reset_accounts'] = FALSE;
after sign in the system immediately.