The Android Nougat Beta Program has officially concluded, according to a change on Google’s landing page for the early access program. All users of the current Beta Program will be updated to the latest public version of Android Nougat for their device and Google will let us know when the Android O Beta Program kicks off.
The second Android O developer preview is scheduled to land sometime in May — more than likely during Google I/O which kicks off on May 17 — and we assume the Beta Program will begin with it. The first dev preview of Android O is listed on the Android Developer site as an alpha release and the second version in May as a beta, hence the logic of an I/O Beta Program launch.
While you’ll still be able to flash subsequent developer previews manually, the Android Beta Program makes the entire process easy. Simply visit the enrolment page (when it goes live again), register your device and wait for the new beta to arrive OTA like any other update.