System Upgrade

Ugrade all installed packages

$ sudo apt-get update
$ sudo apt-get upgrade -y

Reboot

$ sudo reboot

Remove unused packages and old kernel versions

$ sudo apt --purge autoremove

Install Ubuntu update tool

$ sudo apt install update-manager-core

Start Upgrade

$ sudo do-release-upgrade

Note: If it shows There is no development version of an LTS available., run sudo do-release-upgrade -d

System Restart

$ sudo reboot

Verify Distro

$ lsb_release -a

Enabled disabled 3rd party repo

While upgrading distro 3rd party repositories can be disabled by Ubuntu for system stability, and these can be enabled again by looking into /etc/apt/sources.list file.

Source: CyberCiti