The Windows 10 Mobile Build 10136 may get released tomorrow, it seems.
@GabeAul Sorry to get you excited for a build today, but we really needed your input to make the call here.
— Gabriel Aul (@GabeAul) June 15, 2015
But you need to roll-back to Windows Phone 8.1 using Windows Phone recovery
It seems the latest Lumia software recovery tool update may have made it ready for Windows 10 for Phones preview too. If you remember the tool came handy in rolling back the device's software to stable and officially available versions when many preview for developers user faced issue with the
Seems, AT&T which has started releasing Lumia cyan to Lumia 1520 in US is still not making it available to Lumia 1520 with Dev Preview. That may be due to the known issue of Devices running Dev Preview and BitLocker encryption, where you need to roll back your device's firmware