I heard big news when the new Zorin hit the tables, but I was waiting for an upgrade popup. Nothing happened. Latest updates crashed ZorinOS in some critical functions like sleep - btusb crashed and sound could not be restored until I manually reset it using
sudo modprobe btusb
Then just restart the re/enable the the bluetooth service using systemctl
The option to perform a Direct Upgrade from Zorin OS 15.3 to Zorin OS 16 is still in development.
However, you can upgrade to Zorin OS 16 by downloading and installing Zorin OS 16 on your system.