From the Ubuntu wiki on upgrading
Upgrading from Ubuntu 14.04 LTS or 15.10
To upgrade on a desktop system:
To upgrade on a server system:
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.