This is more to aid my memory than anything else, as it is something we do for the 300+ servers on a regular basis that I help to host.

sudo apt-get updatesudo apt-get upgradesudo apt dist-upgradesudo apt-get autoremovesudo apt install update-manager-coresudo do-release-upgrade

Once upgrading to Ubuntu 18 it is good to note that Apache and PHP will stop working. The fix for this is to re-install the php and libapache2 php mod after the upgrade

sudo apt install phpsudo apt install libapache2-mod-php

Some of the common issues are that when you try to do the upgrade you get the error: Please install all available updates for your release before upgrading. To solve this, run

sudo apt-get --with-new-pkgs upgrade