sudo apt clean
To remove all stored archives in your cache for packages that can not be downloaded anymore (thus packages that are no longer in the repository or that have a newer version in the repository).
sudo apt autoclean
To remove unnecessary packages (After uninstalling an app there could be packages you don't need anymore)
sudo apt autoremove
To delete old kernel versions
dpkg --get-selections | grep linux-image
sudo apt remove --purge linux-image-X.X.XX-XX-generic linux-image-X.X.XX-XX-generic