As with most upcoming operating systems, BlackBerry has decided to release a beta 10.1 OS for developers to start testing their apps on. The BlackBerry 10.1 Developer Beta OS is available for the BlackBerry Dev Alpha B, Dev Alpha C, and the BlackBerry Z10.
As always, this OS is a beta OS meant exclusively for developers, not regular consumers. As the Inside BlackBerry Developer's Blog points out,
Note: This SDK OS update is only for testing purposes and does not replace your full BlackBerry 10 OS. To revert your BlackBerry Z10 to a public OS release, please follow the steps outlined in this KB Article. Using BlackBerry Link, you will be able to reload your device software to a full OS and, if needed, download any OS updates from your carrier.