Shortly after the first Developer Preview for Android 8.1 went live, the update has started rolling out over the air (OTA). So those who have enrolled in the Android Beta Program should expect the update notification soon (if not received already). It's worth mentioning that aside from OTA, the only other option to get the beta build currently is to download and flash a factory image - there's an ongoing problem with side loading OTA images, and Google seems to be working on it. Source 1 2
Source: https://www.gsmarena.com/android_81_ota_rollout_has_begun-news-27952.php
