With yesterdays official announcement of Android 5.1, Google has begun rolling out Factory Images for Nexus devices, as well as pushing Android 5.1 source code to the Android Open Source Project repository. Part of that build is giving developers access to any new APIs included in the Android 5.1 release, which is now at API level 22.
The Android Developers page has been updated with the latest release information which lists the API level and outlines some of the new updates included in the new release. The headline feature is the Dual-SIM capability, as well as new enterprise features to better support the launch of Android for Work.
If you’re an Android Developer you’ll need to update your Android Studio install. If you’re not clear on how to do this, Google has a guide on how to update the software.
In the release notes, Google also confirmed which Nexus devices will receive the Android 5.1 update: