I'm not sure its been released to any Lumia officially other than the new ones that shipped with it.
(The people who have it used a sort of workaround via the Windows Insider program where sometimes, instead of upgrading straight to 10, it'll upgrade to Update 2 AND THEN 10. But one can hard reset the phone and stick with Update 2.)
Well, the carriers do slow down the update process typically, but I don't think they have the update to release yet.