Step 1: Update Ubuntu 16.04 Server
sudo apt update && sudo apt dist-upgrade && sudo apt autoremove

Step 2: Install Ubuntu Update Manager
sudo apt-get install update-manager-core
sudo nano /etc/update-manager/release-upgrades
Confirm that the Prompt is equal lts

Step 3: Perform Ubuntu 18.04 LTS Upgrade
sudo do-release-upgrade -d