If you want this across the entire site, you would need to alter the Default.aspx.vb file in the Page_Load event like so:

' add this to the end of the Page_Load event
Dim ph As PlaceHolder = Page.FindControl("CSS")
If Not ph Is Nothing Then
    ph.Controls.Add(New LiteralControl("<base target=""_top" />"))
End If

If this is just for some of the pages, you can add the link tag in the Page Settings.

<base target="_top" />