BlueOS is a ground-up rewrite software to replace Companion. To use it you'll need to download and flash an SD card. It is compatible with both Raspberry Pi 3 and Raspberry Pi 4.
The latest available stable version is:
We recommend using a fresh SD card, with at least 4GB capacity.
The first boot may take a couple of minutes, as it expands the filesystem to the new SD card capacity. It should take around 2 minutes for a 16GB class 10 SD card.
Once BlueOS is installed, updating to a different version is simple via the Version Chooser.
For developers with alternative hardware, or who would rather install over a pre-installed base operating system / image, BlueOS provides an install directory with utilities to help perform manual/software-based installations.