If you run a curl on your website similar to the curl below, does it return nginx versions?
This is NOT secure. We will now make a simple change to the nginx.conf file to hide this

curl -LIsX GET lukeslinux.co.uk | grep nginx

To hide the nginx value, add the following line in the http section

server_tokens off;

Restart nginx and you are done. Test again by rerunning the curl command.