Android 8.1 Developer Preview is now available for download. The members of the Android Beta Program also have the option to wait until the over-the-air update rolls out.
You’ll then receive updates over-the-air to the latest preview releases. The developer preview supports a range of hardware devices, from phones to tablets and TV. This Android 8.1 preview comes in the form of a flashable .zip file that you’re going to have to install yourself.
You can read more details here.