The unsupported operating systems versions will no longer receive security updates. If keeping the outdated version, some unauthorized people may target those outdated operating systems because of their vulnerability, allowing them to breach the server and gain personal information. The best possible way to prevent them is to update your operating system to the latest version regularly.
The below steps can be followed to update the version for Ubuntu & Debian.
*Please make sure to take proper backup of your entire server before performing the OS upgrade.

1. To check the current OS version in the server.

# cat /etc/issue

2. Upgrade the installed packages of Ubuntu version 16.04.

# sudo apt-get update && sudo apt-get upgrade

3. Install ubuntu update manager.

# sudo apt install update-manager-core

4. Start to upgrade Ubuntu server.

# sudo do-release-upgrade

Enter Y to continue.
Once this done, need a system reboot. Enter Y to restart the server.

5. Now you can check the OS version in the server.

# cat /etc/issue