Store checked checkbox in gridview in database ASP.NET C#

I am able to retrieve out data from database to show which is Y and N using checkbox in gridview. However now i want to to able to store which checkbox is checked back into the database after editing.

What i did so far:

.aspx

 <asp:GridView ID="SiteGridView" runat="server" CssClass="datagrid" 
                        GridLines="Vertical" AutoGenerateColumns="False" Width="100%" 
                        AllowPaging="True" AllowSorting="True" 
                        DataKeyNames="promoID"  OnRowCommand="GvPage_RowCommand" 
                        OnPageIndexChanging="GvPage_PageIndexChanging" 
                        OnSorting="GvPage_Sorting" 
                        onrowdatabound="SiteGridView_RowDataBound" OnRowEditing="SiteGridView_RowEditing">
                        <Columns>       
   <asp:TemplateField HeaderText="Default">
                                <ItemTemplate>
                                    <asp:CheckBox ID="process_flag" runat="server" Checked='<%# bool.Parse(Eval("PROCESSED").ToString()) %>' Enable='<%# !bool.Parse(Eval("PROCESSED").ToString()) %>' />
                                  </ItemTemplate>
                                 <ItemStyle Width="20%" />
                            </asp:TemplateField>
   </Columns>
                </asp:GridView>

CodeBehind:

 SqlCommand cmd = new SqlCommand("SELECT * , CAST(CASE defaults WHEN 'Y' THEN 1 ELSE 0 END AS BIT) AS PROCESSED FROM Promotions"); 
            SqlDataAdapter da = new SqlDataAdapter(); 
            DataTable dt = new DataTable(); 
            da.SelectCommand = cmd; 

            // Save results of select statement into a datatable 
            da.Fill(dt);

            SiteGridView.DataSource = dt;
            SiteGridView.DataBind();