In nginx, you can rewrite incoming requests for a naked domain ”example.com” to ”www.example.com” just by using a few lines. It’s so simple. I’ll show you how to do that properly.

server {
  server_name example.com;
  return 301 $scheme://www.example.com$request_uri;
}
server {
  server_name www.example.com;
  * other config 
}

second way

server {

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

server {
listen 80;
server_name www.example.com;
* other configuration options 
}

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Nginx Wiki

Common Nginx Pitfalls