Magento – Magento Installation Error – Magento 2

You could actually put it anywhere, what really matters is where you run it from. If you download composer and put it somewhere in your cli PATH then all you need to do is cd to your magento directory and then run "composer update", however you could also put it in the magento directory and run it from there.

You may already know but just to be clear. The CLI tool doesn't actually have anything to do with running the Magento application. The main thing Composer takes care of is downloading and installing the dependencies of the project, it will also create an autoload file so that those dependencies can easily be loaded from the application.

cd /var/www/html (magento root)
rm -Rf var/generation/* 
rm -rf var/di/*
bin/magento cache:clean

Now you will get 500 error in the website.

chmod -R 777 /var/www/html (magento root)
Add sudo before command if the command does not execute