Как написать инструкцию SQL update для richtextbox?

Hello all. Am new to VB.Net and seeking some assistance. With some help from the internet, I managed to save a richtext document to SQL server and then access for viewing. 

How do I write an update statement to save it back to SQL? 

Thanks in advance.

Dim cmd As New SqlCommand("Select richtext from tablename", con)

       Try
           rtbdoc.SaveFile("Test.rtf")
           Dim Stream = New FileStream("Test.rtf", FileMode.Append, FileAccess.ReadWrite)
           Dim size As Integer = Convert.ToInt32(Stream.Length)
               Dim rtf As [Byte]() = New [Byte](size - 1) {}
               Stream.Read(rtf, 0, size)

               Dim paramRTF As New SqlParameter("@rtf", SqlDbType.VarBinary, rtf.Length, ParameterDirection.Input, False, 0, 0, Nothing, DataRowVersion.Current, rtf)

          cmd.CommandText = "Update table SET firstname = '" & txtfirstname.Text & "', lastname = '" & txtlastname.Text & "' richtext = @rtf where id = @id"

           If con.State = ConnectionState.Open Then con.Close()
           con.Open()

           cmd.Parameters.Add("@id", SqlDbType.Int).Value = txtid.Text
           cmd.Parameters.Add(paramRTF)
           'cmd.ExecuteNonQuery(
           cmd.ExecuteNonQuery()

           MessageBox.Show("File Updated Successfully")
           clear()
       Catch ex As Exception
           MsgBox("Error updating record!", MsgBoxStyle.Critical)
           clear()
       Finally
           If Stream.Null IsNot Nothing Then
               Stream.Null.Close()
           End If
       End Try