If you don’t want to do an in-place upgrade, you can download the ISOs image from its download page and upgrade from the CD/DVD or do a clean install.
To upgrade Ubuntu, please follow the steps below Before upgrading to Ubuntu 17.10, you must first update the current 17.04.
To my surprise, zzupdate utility upgraded my Ubuntu 16.04 LTS to Ubuntu 17.04 (which I never expected). To install the screen tool, run: Note for those upgrading Ubuntu Desktop 16.04 LTS version: Ubuntu 16.04 LTS is the long term support release.
I guess it is because Ubuntu 16.10 is already reached end of life. remote: Total 57 (delta 0), reused 0 (delta 0), pack-reused 57 Unpacking objects: 100% (57/57), done. So it will only search for the next available LTS version, which is Ubuntu 18.04 LTS.
Ubuntu 17.10 (Artful Aardvark) will be released on October 19, 2017.