Microsoft started rolling out another update to their most awaited phone OS i.e. Windows Phone 8.1 having a build no. 12400 released as Developer Preview. If you did not receive the update notification yet, go to phone settings and force a phone update.
Start enjoying the major improvements and functionalities available in Windows Phone 8.1 OS, if you already have WP8 hardware device.
Not yet sure, what are the new changes available in this build after the build version 12397, but there must be some major bug fixes before the final release.
While installing the new update if you face Blue Screen Of Death (BSOD), just do a soft reset of your hardware and it will start installing the OS as expected.