Android (root, 5.0+): Unfortunately, the Android N preview is only available for a few devices. However, you can try out a few of the newest features on Android Lollipop and up with this Xposed module.
Android N-ify aims to bring several of the smaller, more useful features of Android N to older, rooted devices. Currently, it supports two minor but useful tweaks: it can display subtitles in the Settings app, and it enables double-tapping the Recents button to quickly switch apps. In the future, the developer plans to include the improvements to Doze mode, Night Mode and the new notification layout. If you want to start using it already, you can download Xposed and get the module at the source link below.