composer you are using composer 2 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