rebuild composer.json from vendor folder code example

Example 1: update particular package composer

composer update doctrine/doctrine-fixtures-bundle

Example 2: update 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