Try this:
Open a terminal,
Press Ctrl+Alt+T
Run it:
exec sudo -i
apt-get update
apt-get dist-upgrade
apt-get autoremove
apt-get clean
nano /etc/update-manager/release-upgrades
In the open file, leave this line as:
Prompt=lts
Ctrl+O Save file
Ctrl+X Close nano
Reboot your system to finish installing updates, and to upgrade to new available version:
Open a terminal,
Press Ctrl+Alt+T
Run it:
exec sudo -i
update-manager -d
Software Updater will show up and search for the new release, after a few seconds, you will see a screen like below that saying:
However, Ubuntu 16.04 is available now (you have 15.10).
Press the button Upgrade to start upgrading to Ubuntu 16.04.
The Software Updater will ask you to confirm still you want to upgrade, press Start Upgrade to begin installing Ubuntu 16.04.
Now, the Software Updater will prepare to start setting up new software channels, and after a few minutes, the software updater will notify you the details the number of packages are going to be removed, and number of packages are going to be installed, press Start upgrade to continue.
Ended the dist-upgrade, reboot your system and:
Open a terminal,
Press Ctrl+Alt+T
Run it:
exec sudo -i
dpkg --configure -a
apt-get -f install
apt-get -m install
apt-get update
apt-get dist-upgrade
apt-get autoremove
apt-get clean