The best way to redirect non-www to www URL in Nginx according to documentation is to define a separate server for example.org and redirect permanently to www.example.org, here is an example:

server {
    listen       80;
    server_name  example.com;
    return       301 http://www.example.com$request_uri;
}

server {
    listen       80;
    server_name  www.example.com;
    ...
}

Restart and verify config using following commands(for CentOS):

Reload:
systemctl restart nginx
Verify:
systemctl status nginx