Android 8.1 system images are available to download for the above mentioned devices, but that requires an unlocked bootloader. There's also an other option, you can enroll your device in automatic OTA system updates through the Android Beta Program. Once enrolled, your device will receive regular over-the-air (OTA) updates of all milestone builds in the Android 8.1 Developer Preview. When the next version of Android is released, your device will automatically update to the final version.
You can also un-enroll your device at any time. The device will receive an OTA update to the most recent production version of Android available for that device (for example, Android 7.1 Nougat). The update requires a full device reset, so user data on the device will be removed.