apt-get(8) upgrade
APT package handling utility - - command-line interface
    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.
source manpages: apt-get