How to change ie version from htmldocument?

I want to change ie version form HtmlDocument:

 HtmlElementCollection metas = Document.GetElementsByTagName("META");
            foreach (HtmlElement meta in metas)
            {
                if (meta.GetAttribute("content") == "IE=9.0000")
                {
                    meta.SetAttribute("content", "IE=8.0000");
                    break;
                }
            }

but the it is still ie9 after changing, why?

Awen