How to edit data tabale use to bind the gird view

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C#

ASP.NET

Hello,
I take a data table to bind grid view.
It after clicking on add button it inserted data into grid view successfully.
I take link button in grid view to edit and delete.
on click on delete it deleted data of selected row.
On click on select(i.e edit) it assign a value to drop down list and text boxes successfully.
after clicking on edit and changing values on can again click on add button .At this time I want to Update the selected row with new values,
I cannot able to solve this problem
Can any one help me please.
Thanks in advance.

My code is as follow

<asp:GridView ID="grdvPart" runat="server" AllowPaging="True" AllowSorting="True" CssClass="table table-bordered"
                            AutoGenerateColumns="False" CellPadding="0"  GridLines="None"  Width="100%" ShowFooter="True" OnRowCommand="grdvPart_RowCommand" >
          <Columns>
          
                 <asp:BoundField DataField = "prt_id" HeaderText = "Shape Part code" />
                 <asp:BoundField DataField = "prt_nm" HeaderText = "Parts" />
                 <asp:BoundField DataField = "shpprtQuantity" HeaderText = "Quantity" />
                <asp:TemplateField HeaderText="">
                    <ItemTemplate>
                        <asp:LinkButton  ID="lbtnSelect" runat="server" Text = "Select" CommandArgument='<%# Eval("prt_id")%>' CommandName="SelectRecord"></asp:LinkButton>
                         </ItemTemplate>
                   </asp:TemplateField>    
                   <asp:TemplateField HeaderText="">
                    <ItemTemplate>
                        <asp:LinkButton ID="lblremove" runat="server" Text = "remove" CommandArgument='<%# Eval("prt_id")%>' CommandName="DeleteRecord"></asp:LinkButton>
                    
                        </ItemTemplate>
                        
                </asp:TemplateField>               
               </Columns>
         </asp:GridView>

Code Behind

on page load{
DataTable dt = new DataTable();
                dt.Columns.AddRange(new DataColumn[3] { new DataColumn("prt_id"), new DataColumn("prt_nm"), new DataColumn("shpprtQuantity") });
                ViewState["Part"] = dt;
this.bindDetails();
}

  private void bindDetails()
        {
            grdvPart.DataSource = (DataTable)ViewState["Part"];
            grdvPart.DataBind();
}

protected void btnAdd_Click(object sender, EventArgs e)
        {
            DataTable dt;
            dt = (DataTable)ViewState["Part"];
 dt.Rows.Add(tbxPCode.Text, ddlPart.SelectedItem.Text, tbxQuantity.Text);
  ViewState["Part"] = dt;
this.bindDetails();
            clear1();
        }
   protected void grdvPart_RowCommand(object sender, GridViewCommandEventArgs e)
        {
if (e.CommandName.Equals("DeleteRecord"))
            {
 DataTable dt = (DataTable)ViewState["Part"];
if (dt.Rows.Count >= 0)
                        {

                            dt.Rows.RemoveAt(Convert.ToInt16(grdvPart.SelectedRow));
                            this.bindDetails();
                        }
}
  if (e.CommandName.Equals("SelectRecord"))
            {
              
                    string code = grdvPart.Rows[Convert.ToInt32(grdvPart.SelectedRow)].Cells[0].Text;
                    tbxPCode.Text = code;
                    ddlPart.SelectedItem.Value = code;
                    ddlPart.SelectedItem.Text = grdvPart.Rows[Convert.ToInt32(grdvPart.SelectedRow)].Cells[1].Text;
                    HiddenFieldpart.Value = id.ToString();
}