I've just done an upgrade of my lubuntu. I see for example that I have in the system old linux kernels. How can I clean and tidy my OS?
If you like the command line, you can try
sudo apt autoremove to get rid of some of the old kernels.
If you prefer a GUI, you could try Ubuntu Cleaner, a fork of the crud-cleaning portion of the late lamented Ubuntu Tweak application. From the linked article, you can install it from a ppa by issuing these commands:
sudo add-apt-repository ppa:gerardpuig/ppa
sudo apt-get update && sudo apt-get install ubuntu-cleaner