$node->panelizer->view_mode = 'press_release';
$node->save();

string(4) "news"
array(2) {
  [0]=>
  array(3) {
    ["view_mode"]=>
    string(4) "news"
    ["default"]=>
    string(4) "news"
    ["panels_display"]=>
    array(0) {
    }
  }
  [1]=>
  array(3) {
    ["view_mode"]=>
    string(4) "full"
    ["default"]=>
    string(13) "press_release"
    ["panels_display"]=>
    array(0) {
    }
  }
}

$panelizer_arr = $node->panelizer->getValue();
    $panelizer_arr[1]['default'] = 'test_display';
    $node->panelizer = $panelizer_arr;
    $node->save();