Composer update dont work

[APdevelopments]

I need to run composer update at my hosting so I log in with ssh and try to run comand:

composer update

inside /www folder where I have laravel and composer instalation

but I get error: http://i.stack.imgur.com/FGC01.png

in contact with my hosting provider they tell me to run command:

php -d memory_limit=512M composer update

I run this command but I get: "Could not open file: composer"

What to do? What is the soluton here?