upgrade the composer to version 2.0 in ubuntu code example

Example 1: ubuntu install composer

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '8a6138e2a05a8c28539c9f0fb361159823655d7ad2deecb371b04a83966c61223adc522b0189079e3e9e277cd72b8897') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
php composer-setup.php
php -r "unlink('composer-setup.php');"

Example 2: 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