Re: how can i upgrade my Lumia 1320 on windows 10
You would need to enroll your device in the Insider Program. To do that, you need to download the app "Windows Insider" from the Windows Phone store and follow the instructions as given in the app.
But considering that you apparently don't know much about that, I suggest you keep WP 8.1 on your device and wait for the official rollout to start, which is probably beginning in January 2016. Windows Insider provides you with early versions, also referred to as beta versions. They most certainly contain bugs and may even be unstable and unreliable. Though the latter 2 don't really apply anymore, since the latest few builds have been almost rock solid. But there are definitely quite a lot bugs left. So you better be patient.