UPDATE  table_name  SET  forum=300  WHERE  forum=200;

$customer = Account::find()
                 ->select(['salt_activ'])
                 ->where(['forum' => 200])
                 ->one();
                  $customer->forum = 300;
                  $customer->update();

$customer = Account::find()
     ->where(['forum' => 200])
     ->one();
$customer->forum = 300;
$customer->save();