add_filter( 'frm_global_setting', 'replace_global_setting', 10, 3 );
function replace_global_setting( $setting, $string, $settings ) {
    $form_id = $settings->current_form;
    if ( $form_id === 5 && $string === 'login_msg' ) { // change 5 to your form id and login&msg to the string name.
       $setting = 'please log in'; // set the message here
    }
    return $setting;
}