I dont understand when/where you want to set cssclass..
If you want to set it in ItemDataBound:
protected void Repeater_ItemDataBound(object sender, RepeaterItemEventArgs e)
{
LinkButton my_btn = (LinkButton)e.Item.FindControl("my_btn");
if (my_btn != null) my_btn.CssClass = "someCssClassHere";
}
or if you want to set after Click:
protected void cmdSelect_click(object sender, CommandEventArgs e)
{
LinkButton my_btn = (LinkButton)sender;
my_btn.CssClass = "someCssClassHere";
}