See here if your SEL is still on mono 3.2.8 and you are trying to get to mono 5.
SSH ONLY and on Systems with 3.2.8 where original Mono 5 SEL script is not working
Backup your system before making any changes in the Linux OS.
Login with root
nano sources.list to find out which version of Lubuntu. It will be repeated toward the end of each source line (usually trusty) then exit
EXAMPLE (oneiric repeats)
There should be a mono sources list file in here
Nano that file and it should contain something like "deb http://download.mono-project.com/repo/ubuntu VERSION main" (VERSION could be wheezy, trusty, bionic, oneiric or something else)
Make sure VERSION matches what was repeated in the sources.list and add "-stable"
EXAMPLE "deb http://download.mono-project.com/repo/ubuntu trusty-stable main"
Save and Exit
Now run apt-get update and apt-get upgrade.
If upgrade is not working/completing, you can run apt-get dist-upgrade. The overall upgrade should take 5-10 minutes. You will see mono 5.XX in the file names as it unpacks and installs.
Users may have to fix some broken events after as this is a big upgrade.