You should be able to call a javascript function and change the value of the textbox.
Like,
<asp:HyperLink runat="server" onClick="clientMethod('<%# Eval("valuetoupdate") %>');">click </asp:Hyperlink>
And, write the client-side javascript function like this:
<script language="javascript">
function clientMethod(value) {
document.getElementById("txtBoxID").value = value;
}
</script>
Or even use the itemcommand for server-side function call:
<asp:TemplateField>
<ItemTemplate>
<asp:Hyperlink ID="hlSample" runat="server" CommandArgument='<%# Eval("valueToUpdate") %>' CommandName="Update" OnCommand="itemcommand" />
</ItemTemplate>
</asp:TemplateField>
And , then in itemcommand event:
protected void itemcommand(object sender, CommandEventArgs e)
{
//Use (e.CommandName == "Update" ) to determine the action,
//and then use the e.CommandArgument to retrieve the value to be updated
}