gridview edit template update won't work

I have this gridview. I want it be able to do select, delete, and edit.
After I click on edit, it shows update imagebutton, cancel imagebutton, and the textbox.
However, after I click on "update", it won't update.
Anybody knows why? Code is below.1 <asp:GridView ID="gvCustomer" runat="server" AllowPaging="True" AutoGenerateColumns="False"
2 CellPadding="4" DataKeyNames="CustomerID,Customer" DataSourceID="SqlDataSourceCustomer"< ...