We don’t provide support for 18.04, and it being the last LXDE release didn’t have a supported upgrade path.
- https://lubuntu.me/bionic-eol/
- Lubuntu 18.04 LTS is nearing it's EOL (end-of-life)
- Lubuntu 18.04 LTS End of Life (30-April-2021)
thus I’ve moved this to off-topic.
A re-install is what I’ll suggest too, given 18.04 was so long ago (we did have some non-published instructions I’d point people to on achieving a release-upgrade, but those no longer exist).
Your system is not receiving security updates so keep your system off-line, unless you’ve enabled ESM or extended support which is a service offered by Canonical. Canonical offer support (paid/contract), and most generic GNU/Linux sites will support you, however Ubuntu support has ended for 18.04.