sudo mkdir /var/www
sudo chown wordpress. /var/www
mv /usr/share/nginx/www/* /var/www
mv /usr/share/nginx/www/.* /var/www
sudo usermod -a -G wordpress youruser
sudo chgrp -R wordpress /var/www
sudo chmod -R g+w /var/www
Hope you can help me, I have the same problem I don't have writing permissions to update some files on wordpress using Winscp, I've followed this tutorial to install wordpress "https://www.digitalocean.com/community/tutorials/how-to-set-up-wordpress-multisite-with-nginx-on-ubuntu-14-04" and works fine.
I've installed wordpress under "/usr/share/nginx/wpdir", when I try to run as you mention "sudo usermod -a -G wpdir myuser" I get the message "group 'wpdir' does not exist".
I have no user root instead I've created a new user "myuser" with sudo rights following this tutorial: https://www.digitalocean.com/community/tutorials/initial-server-setup-with-ubuntu-14-04
I have another question: why it is important to move wordpress from: "/usr/share/nginx/wpdir" to "/var/www"