If you have your nginx server setup with valid SSL certificates to support https, you can redirect all http incoming requests to https with a simple 301 ‘Permanently Moved’ response to the client.
Returning a 301 is considered best practice for enforcing an http to https upgrade.
This post covers this approach, which involves configuring a server block for port 80 to always respond with a 301 response including the https url:
return 301 https://www.example.com$request_uri;
The example config in the above article also shows how to redirect first to your server defined with no subdomain, and then additionally redirect from the request with no subdomain to your www subdomain, e.g from http://www.example.com to https://example.com then to https://www.example.com. Apparently the way nginx caches server names there’s performance advantages to redirect to a no-name subdomain first. Redirecting directly to your www subdomain obviously still works too.