Launch your terminal and copy the commands to the command line one at a time:
sudo apt update sudo apt install lubuntu-desktop
sudo apt install --no-install-recommends lubuntu-desktop
if you want a lighter installation.
Removing other X/K/Ubuntu components after upgrading to Lubuntu
Adding Lubuntu to your system will keep all the applications from your existing system.
This will not delete any of your own data from those applications. If you later wish to re-add them, your data will be there waiting for you.