public ActionResult Edit(int id, User source)
{
    User existing = db.Users.Single(x => x.Id == id);

    if (existing != null)
    {
        existing.Username = source.Username;
        existing.EmailAddress = source.EmailAddress;
        existing.Password = source.Password;

        db.Entry(existing).State = System.Data.Entity.EntityState.Modified;
        db.SaveChanges();

        return View("Index");
    }
    else
    {
        // report user not found
    }
}