How to upgrade ubuntu 16.04 to ubuntu 18.04

Check Ubuntu version before upgrading.

    lsb_release -a

First, we have to run an update

    sudo apt update

Then run upgrade command

    sudo apt upgrade

After that dist-upgrade

    sudo apt dist-upgrade

Then update core manager

    sudo apt install update-manager-core

Then edit below file

    sudo vim /etc/update-manager/release-upgrades

At the end of the file set

    Prompt-lts

Then save this file

Then do a release upgrade

    sudo do-release-upgrade -d

Once done, Then restart the machine and check again.