I can't find the option anywhere.
You can access the new Win10 menu by clicking the shortcut on the Start8 menu at the top left.
If you mean you want the Windows 8 fullscreen menu look, Microsoft have removed this from Windows 10, though you can make their new smaller menu fullscreen if you wish.
Ah, I wasn't aware they'd removed the full-screen Modern UI and moved it to the new Start menu instead