Return a partial view from a controller?

When the partial takes a different model than the method you're including it in you need to use the overload that takes a model parameter and supply the model for the view. By default it uses the same model as the including view. Typically you only need the path if it's in a different, non-shared folder. If it's in the same controller's folder, using just the name should do the trick.

@Html.Partial("_Address", Model.Address)

On the other hand, if you're asking how do I get the partial view from an action included in my page, then you want to use the Action method instead of the Partial method.

@Html.Action("Address")

EDIT

To make the partial work you need to pass a Contact model to the contact view.

public ActionResult Contact()
{
     var contact = new Contact
     {
        Address = new Address
                  { 
                       Line1 = "111 First Ave N.",
                       Line2 = "APT 222",
                       City = "Miami",
                       State = "FL",
                       Zip = "33133"
                  }
     }

     return View(contact);
}

demo for you:

    public ActionResult Update(Demo model)
{
    var item = db.Items.Where(item => item.Number == model.Number).First();
    if (item.Type=="EXPENSIVE")
    {
        return PartialView("name Partial", someViewModel);
    }
}