EntityFramework 如何更新 detached 实体

Question

如题，EntityFramework 如何更新 detached 实体，如下代码不能更新：

Product p = null;
using (EFDbContext context = new EFDbContext()) {
    p = context.Products.First();
}
using (EFDbContext context = new EFDbContext()) {
    p.Remark = "aaa";
    context.SaveChanges(); //不能更新
}