Today’s the big day: when Google starts to push out Android 6.0 Marshmallow to devices with an over the air update as well as posting the open source code to the Android Open Source Project (AOSP).
The first part is the factory images. You can flash the appropriate image onto your device to get the software update if you don’t want to wait for an OTA update. Directions for flashing as well as the factory image for the Nexus Player are at this link.
There don’t seem to be official plans from other Android TV manufacturers about updating their devices to Marshmallow, but once the platform code is uploaded it shouldn’t be too long. Android TV is more restrictive as to what can be changed, and a lot of the platform can be updated individually through the Play Store, so companies like Razer or Nvidia shouldn’t have to do very much.