How to Upgrade Ubuntu 15.10 to Ubuntu 16.04 LTS
These release notes for Ubuntu 16.04 LTS (Xenial Xerus) provide an overview of the release and document the known issues with Ubuntu 16.04 LTS and its flavours.
Ubuntu 16.04 LTS will be supported for 5 years for Ubuntu Desktop, Ubuntu Server, Ubuntu Core, and Ubuntu Kylin. All other flavours will be supported for 3 years.
Upgrade to Ubuntu 16.04
$ sudo apt-get update
$ sudo apt-get upgrade
Now install the update-manager-core package on your system.
$ sudo apt-get install update-manager-core
Now edit /etc/update-manager/release-upgrades file and make sure Prompt value is set to lts like below.
Finally execute below command to upgrade your Ubuntu 14.04 LTS system to Ubuntu 16.04 LTS system.
$ sudo do-release-upgrade
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04 LTS