try this.
if(ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "javascript:if(confirm('This PassCode Already Exists!Are you Shore you want to update this.') == false) return false;", true))
{
p.UpadteIssuePass(ddlpasscode.SelectedValue, Convert.ToInt32(ddlname.SelectedValue), Convert.ToInt32(txtcounting.Text),
crtby, con, tran);
}