update data on button click using linq in asp.net Entity Framework

Question

I am trying to do update data...i use linq but cant take

 

FirstOrDefault()

 

First()

protected void BtnUpdate_Click(object sender, EventArgs e)
{
using (var context = new WallpaperEntities7())
{
var CategoryName = TxtCategoryName.Text;
var Category = int.Parse(Request["Id"])
WallPaperCategory std = (from x in context.WallPaperCategries
where x.CategoryName == CategoryName
select x)