How to upgrade from 14.04 LTS or 15.10 to 16.04 from terminal

The easiest way is sudo do-release-upgrade.

You can use sed for editing your sources.list file. I suggest you to backup it first.

To upgrade on 15.10:

sed -i -e "s/trusty/wily/g" /etc/apt/sources.list
apt-get update && apt-get dist-upgrade

To upgrade on 16.04:

sed -i -e "s/wily/xenial/g" /etc/apt/sources.list
apt-get update && apt-get dist-upgrade

Try this first do:

sudo nano /etc/apt/sources.list

then change everything with trusty in it to either xenial (for 16.04) or wily (for 15.10)

then do:

apt-get update;apt-get upgrade -y

and then there you go, you have a fully upgraded 15.10/16.04 system, you do need to run

apt-get autoremove --purge -y;apt-get clean

afterwards