Upgrading from Ubuntu 14.04 LTS or 15.10

14.04 LTS to LTS upgrades will be enabled with the 16.04.1 LTS point release, in approximately 3 months time.

To upgrade on a desktop system:

• Open the “Software & Updates” Setting in System Settings.

• Select the 3rd Tab called “Updates”.
• Set the “Notify me of a new Ubuntu version” dropdown menu to “For any new version” if you are using 15.10, set it to “long-term support versions” if you are using 14.04 LTS.
• Press Alt+F2 and type in “update-manager” (without the quotes) into the command box.
• Software Updater should open up and tell you: New distribution release ‘16.04 LTS’ is available.
• Click Upgrade and follow the on-screen instructions.

To upgrade on a server system:

• Install the update-manager-core package if it is not already installed.

• Make sure the /etc/update-manager/release-upgrades is set to normal if you are using 15.10, lts if you are using 14.04 LTS.

• Launch the upgrade tool with the command sudo do-release-upgrade.

• Follow the on-screen instructions.

Note that the server upgrade will use GNU screen and automatically re-attach in case of dropped connection problems.

There are no offline upgrade options for Ubuntu Desktop and Ubuntu Server. Please ensure you have network connectivity to one of the official mirrors or to a locally accessible mirror and follow the instructions above.

Note : If you are using Ubuntu 15.04 then you need to upgrade to ubuntu 15.10 then 16.04.

For more information follow ubuntu official website https://wiki.ubuntu.com/XenialXerus/ReleaseNotes