I've had to upgrade a couple of Ubuntu servers recently. Here are 4-commands I usually use.
1) First: Update the current release list with latest versions of all packages:
Command: apt-get update
2) Second: Upgrade all installed packages to their latest available versions:
Command: apt-get upgrade
In this step, user is prompted for a list of upgrades and I just answer 'yes'-Y for all prompts
3) Third: Next is to upgrade all dependencies, adding / removing packages, and any other upgrade not settled by step#2 above
Command: apt-get dist-upgrade
4) Lastly: Once the current system is updated, this commands upgrades it to the next release
All the above have usually taken about 30minutes.
At the end of your installation, check your release:
command: lsb_release -a
No LSB modules are available
Distributor ID: Ubuntu
Description: Ubuntu 16.04.2 LTS
If at this point you check for a new release with command:do-release-upgrade...again,
You should get the message: No New release found.
Thats all Folks!!