The first upgrade from Windows 7 or 8.1 to the Preview is done via the Windows Insiders Page, if your doing it online. That's what's shutdown at the moment. You can sign up you just can't install the Preview from there. If your already running the preview, new builds come via Windows update. If you wait till the 29th to upgrade to Window 10 it will likely come via Windows Update.
Last edited by alphanumeric; 18 Jul 2015 at 19:25.