If you registered to test the PS4’s new system software update numbered 3.50 and dubbed “Musashi,” you should check your mail.
Those who have been selected to test the update should have their invite in the mailbox, as Sony Computer Entertainment Europe and Sony Computer Entertainment America just sent them out.
The mail includes a link that displays the usual 12-digit code, that will have to be redeemed in the PlayStation Store. Once you do, you’ll have the option to update to system software 3.50, or back to the current 3.15 version by hitting Settings -> System Software Update.
Just remember that if you share any details about the update, you do it at your own risk, as there’s an non-disclosure-agreement that you need to accept when getting your code.
Update: the post has been updated to reflect the fact that invites have been sent out in North America as well.