Install Ubuntu Updates (command line)

Update ubuntu (keep current version)

Use the following command to install updates and maintaining the major version (if you are running Ubuntu 14.04, it will never upgrade to Ubuntu 16.04).

sudo apt-get update && sudo apt-get upgrade

You could optionally run sudo apt-get dist-upgrade as well.

upgrade: packages are replaced by newer versions, but no package is to be added or removed.

dist-upgrade: will install or remove packages as necessary to complete the upgrade.

I have been using upgrade for a few years now and it seems fairly realiable and doesn't break stuff.

If a new update require a new package, it will report some packages haven been held back. You might want to run sudo apt-get dist-upgrade to resolve the issue.

upgrade
   upgrade is used to install the newest versions of all packages
   currently installed on the system from the sources enumerated in
   /etc/apt/sources.list. Packages currently installed with new
   versions available are retrieved and upgraded; under no
   circumstances are currently installed packages removed, or packages
   not already installed retrieved and installed. New versions of
   currently installed packages that cannot be upgraded without
   changing the install status of another package will be left at
   their current version. An update must be performed first so that
   apt-get knows that new versions of packages are available.

dist-upgrade
   dist-upgrade in addition to performing the function of upgrade,
   also intelligently handles changing dependencies with new versions
   of packages; apt-get has a "smart" conflict resolution system, and
   it will attempt to upgrade the most important packages at the
   expense of less important ones if necessary. The dist-upgrade
   command may therefore remove some packages. The
   /etc/apt/sources.list file contains a list of locations from which
   to retrieve desired package files. See also apt_preferences(5) for
   a mechanism for overriding the general settings for individual
   packages.

Upgrade Ubuntu to newer version

E.g. upgrade from Ubuntu 14.04 to Ubuntu 16.04.

Note: you might want to backup your system before performing an upgrade, and read a more detail guide specific to the version you are dealing with.

sudo apt-get updatesudo apt-get upgradesudo apt-get dist-upgradesudo apt-get install update-manager-coresudo do-release-upgrade