Can't upgrade to 16.04 from 15.10

I just started following this digitalocean article and did every step until the do-release-upgrade part.

However I get

Checking for a new Ubuntu release 
No new release found

I've tried sudo do-release-upgrade as well.

There are 2 ways you can upgrade to 16.04 LTS, using software and update or terminal

Software and update

In software and update window on the 3rd tab you can find the os updates just click install.

Update Ubuntu using software and update

Terminal

Check this article hope that I helps you

For short here are some commads to do upgrade from ubuntu 15.10 wily werewolf to ubuntu 16.04 xenial xerus (LTS), open a new Terminal and type them one by one

sudo apt-get update

sudo apt-get upgrade

sudo apt-get dist-upgrade

at last :

sudo update-manager -d

and click upgrade and continue by choosing upgrade and start upgrade till it starts upgrading