This step is only needed once per machine (per project by default, but globally on machine also possible), for how alternatives see: https://getcomposer.org/download/).
Composer is a command line tool, so main way to install it is via command-line, example with only requirement on php being installed:
php -r "readfile('https://getcomposer.org/installer');" | php
Updating via composer
1. Running composer update and version changes