composer 2 update composer global update code example
Example 1: upgrade composer
To update composer command just find location of composer.phar and update as:
where composer #Retrieves you location of your composer.phar
cd path_to_composer.phar #Move to directory with composer.phar
php composer.phar self-update #Update your composer
Example 2: composer upgrade to 2
run which composer (output /usr/bin/composer)
run php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
run sudo php composer-setup.php --install-dir=/usr/bin --filename=composer