You can update through the Mac App Store. Update: Looks like you can download iTunes 11 from Apple now.
Here are the features that Apple promised in this new version of iTunes when they first announced it:
- Simpler UI
iTunes 11 sports an all new edge-to-edge design, that presents a grid of albums. Just like Folders in iOS, once an album is clicked, it expands inline to reveal a list of the album tracks. Apart form your own, locally stored tracks, you can also see top songs and albums on iTunes by clicking the “in the store” button.
- Up Next
As a part of the new interface, iTunes also introduces a new way to see upcoming songs and also gives you a way to change their ordering. By clicking the Up Next Button in the New Playing section of iTunes, iTunes pops up a window, allowing to manipulate the playing order of upcoming songs.
- Updated Search
The search interface has been updated to show inline results in a popover, unlike the current method where the main iTunes songs list presents search results.
- New Mini Player
The mini player has been revamped with a sleek new design that has minimal controls and an option to search with a similar popeover interface.
- Redesigned iTunes Store (to match iOS)
In line with the redesigned iTunes Store on iOS 6 enabled devices, Apple has also given the desktop iTunes Store a redesign.
- Better iCloud integration for purchases and viewing.
Apple’s also added movie position syncing, so that when you pause a movie on your iOS device or Apple TV, you can resume right from there on your Mac or PC. iCloud also syncs all the movies, music or videos you’ve previewed on iTunes Store across all your devices. iTunes displays a history of these previews in a popover, available as an option in the iTunes Store.
You can update iTunes through Software Update (pre-Mountain Lion) or the Mac App Store.
We’re updating our machines now and we’ll update you with our first impressions—or issues we find.