Code to update Data Adapter

I have a forum where I fill a datagrid with all the data from a table. The table name is pulled from a text box like this (button control):

Code:

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      Dim cnn As SqlClient.SqlConnection = _
                New SqlClient.SqlConnection( _
                "Data Source=ServerName;" & _
                "Initial Catalog=DataBaseName;" & _
                "User ID=SQLUser;password=password")

        Dim strTableName As String = txtTableName.Text
        Dim cmd As New SqlClient.SqlCommand
        Dim da As New SqlClient.SqlDataAdapter
        Dim ds As New DataSet
        Try
            da.Fill(ds, strTableName)
        Catch ex As Exception
            MsgBox("Error: " & ex.Source & ": " & ex.Message, _
                MsgBoxStyle.OKCancel, "Fill Grid Error")
        End Try

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Now I would like to add a second button control to save any changes made in the grid. I thought this would do the trick but I am getting a table mapping error. What is missing?

Code:

---

      Dim cnn As SqlClient.SqlConnection = _
                New SqlClient.SqlConnection( _
                "Data Source=ServerName;" & _
                "Initial Catalog=DataBaseName;" & _
                "User ID=SQLUser;password=password")

        Dim strTableName As String = txtTableName.Text
        Dim cmd As New SqlClient.SqlCommand
        Dim da As New SqlClient.SqlDataAdapter
        Dim ds As New DataSet
        Try
            da.Update(ds, strTableName)
        Catch ex As Exception
            MsgBox("Error: " & ex.Source & ": " & ex.Message, _
                MsgBoxStyle.OKCancel, "Save Udpates")
        End Try

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