Zorin Os technical support recommends doing a clean installation of Zorin Os Ultimate or Zorin Os Ultimate Lite, but in some cases this is not what you want and there is an extremely simple way to do it without the need to reinstall.
The first thing we must do is open a terminal and remove the default configuration of Zorin Os with the following command:
sudo apt remove zorin-os-default-settings
Then we are going to install the Zorin Os Lite packages and the Lite desktop environment, with the following command:
sudo apt install zorin-os-lite-core zorin-os-lite-desktop
During the installation of the desktop, we will be asked to change the visualization manager, so we must choose the visualization manager «lightdm«, Then we simply have to restart our computer.
We will be shown a slightly different start screen which will allow us to log into the XFCE desktop environment, which is the default of the Zorin Os Ultimate Lite version. In this simple way we can go from the full version of Zorin Os Ultimate to the light version, quickly and easily
source: How to install Zorin Lite environment in Zorin Os Ultimate version | From Linux