Problem with sorting the gridview

[Gracie Q]

I'm currently using Gridview to display some data from SQL however having problem with allowing users to sort the data. Here's what I got:

 <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
                                        ConnectionString="<%$ ConnectionStrings:ServiceNowDB %>" 
                                        ProviderName="<%$ ConnectionStrings:ServiceNowDB.ProviderName %>" 
                                        
                                        
                                        SelectCommand="SELECT [number], [request_item], [assigned_to], [state], [subscriber], [room_num], [request_type], [deploy_type], [due_date], [item_sysid], [task_sysid] FROM [sc_tasks_updated] WHERE ([state] NOT LIKE '%' + ? + '%')">
                                        <SelectParameters>
                                            <asp:Parameter DefaultValue="Closed" Name="state" Type="String" />
                                        </SelectParameters>
                                    </asp:SqlDataSource>
                                    <br />
                                    <asp:GridView ID="GridView1" runat="server" AllowSorting="True" 
                                        AutoGenerateColumns="False" CellPadding="4" DataSourceID="SqlDataSource1" 
                                        ForeColor="#333333" GridLines="None" 
                                        EnableSortingAndPagingCallbacks="false">
                                        <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />
                                        <Columns>
                                            <asp:BoundField DataField="number" HeaderText="Number" 
                                                SortExpression="number" ItemStyle-HorizontalAlign="Center">
<ItemStyle HorizontalAlign="Center"></ItemStyle>
                                            </asp:BoundField>

..... more asp:BoundFields.....

</Columns>
</asp:GridView>

code behind:

on page_load, I do some data manipulation like this:

protected void Page_Load(object sender, EventArgs e)
{
string takslink = "http://www.domain.com/task.list?id=2343";

thisRow.Cells[0].Text = "<a href=\"" + tasklink + "\">" + thisRow.Cells[0].Text + "</a>";

thisRow.Cells[4].Text = thisRow.Cells[4].Text.Replace(":", "");

}

When user click on the column title to sort, the page is posted back with the same set of data but all the data manipulation I did on page_load is lost.

Any help is appreciated. I'm a beginner.

Thanks
GQ