how to display data in label based on gridview dropdown onselectedindexchanged

create a event handler like below :

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
// your code
}

then attach the event handler to the dropdownlist like shown below:

<asp:GridView ID="GridView1" runat="server">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true"
onselectedindexchanged="DropDownList1_SelectedIndexChanged">
</asp:DropDownList>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>