We recommend using Composer to manage all third party PHP code dependencies, include contributed Drupal modules and themes. Composer comes pre-installed in the drupal container.
Use composer require to install new packages. Example:
# Install the ctools module. This will fail if ctools requires an update to one of your existing dependencies.
composer require drupal/ctools
# Install the ctools module and update any dependencies ctools has.
composer require drupal/ctools --update-with-dependencies@todo
Use composer update to update existing packages. Packages will be updated according to the version constraints set in composer.json, and considering the version constraints of any other installed packages. It is best practice when updating packages to update one package at a time (with all of it's dependencies). Example:
# Update Drupal core only.
composer update drupal/core
# Update Drupal core and all of the things it depends on.
composer update drupal/core --with-dependencies