There are many ways to upgrade from one version of Ubuntu to another. And one of them is by using ubuntu update manager in your system manager. There is alternative way to upgrade your system to newer stable release of Ubuntu.
But for this method to work you need to install a package and execute following command to do that.
sudo apt-get -y install update-manager-core
Now execute following command to start your system upgrade.
From now on just follow the instructions on the screen. If you want to have additional control over upgrade process then execute above command with --help flag.