Subtract from "device" column within the same row SQL (VB.NET)

Try


          Dim query As String = "select serial from verfication where serial=@serial"
          con.Open()
          cmd = New SqlCommand(query, con)
          cmd.Parameters.AddWithValue("@serial", serialtxtbox.Text)
          Dim dr As SqlDataReader
          dr = cmd.ExecuteReader()

          While dr.Read


              MsgBox("user available")
              con.Close()

              Exit Sub
          End While

          MsgBox("wrong serial")
          con.Close()

      Catch ex As Exception
          MsgBox("Please check your internet connection")

      End Try

While dr.Read


           dr.Close()

           '  con.Open()
           Dim query1 As String = " Update verfication SET device = device - 1 WHERE serial=@serial"

           cmd = New SqlCommand(query1, con)
           cmd.Parameters.AddWithValue("@serial", serialtxtbox.Text)
           'Dim dr1 As SqlDataReader
           dr = cmd.ExecuteReader()

           MsgBox("user available")


           Exit Sub
           End While

Msg 156, Level 15, State 1, Line 1
Incorrect syntax near the keyword 'DECLARE'.
Msg 137, Level 15, State 2, Line 1
Must declare the scalar variable "@adminaccess".