how to update an individual app through terminal

To only upgrade Firefox on its own do:

sudo apt-get dist-upgrade firefox

or

sudo apt-get upgrade firefox

---

from man apt-get

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.

To upgrade only Firefox from the terminal type:

sudo apt upgrade firefox  

The above command works without the --only-upgrade option. If your Firefox is already the latest version the command will not install any new package, and you will get the following message:

firefox is already the newest version.