How to change the temporary upload directory nginx/passenger?

You might want to look at the Nginx upload module. It lets you ask Nginx to write the file to a folder and then pass a pointer to its location to your backend. Temporary upload path can be set using $upload_tmp_path variable. Read more at http://www.grid.net.ru/nginx/upload.en.html

If possible at all, you should definitely use Reza's answer and use the 'upload' module.

However, if you use nginx as a reverse proxy with a backend on a different machine, that module won't necessarily do much for you. To choose where your temporary request bodies (including uploaded files) go, use the client_body_temp_path config option.