The SHIELD Preview Program has now been opened up to everyone. SHIELD TV and SHIELD Tablet owners are now able to register to become part of the program. You will be able to gain access to upgrade software before it is made available to the general public. This allows Nvidia to gain crucial feedback so they can make any necessary changes before releasing updates to the masses.
Signing up for the program is quite simple. You need a SHIELD Forum account, the name you use on that forum, and the serial number and product ID for your SHIELD product. The updates will begin to roll-out to your device as they are available once you are registered. While the program is available for both TV and Tablet owners it is best suited for TV users. In other words you may not see these updates if you are on a Tablet for a while.
via Nvidia