...
Finally stop as many services as possible, especially those not managed by Ubuntu. Examples include web server deaemons (Apache, nginx, etc), database servers (PostgreSQL, MySQL, etc), and any other non-critical services.
If possible obtain console access to the system to perform the actual release upgrade.
Perform the Upgrade
It is preferable to upgrade from the console. Even in the case of cloud computing, most system provide a back door console access via the web browser.
Perform the release upgrade,
...