If you want to redirect traffic to a different subdomain, you can add a rewrite clause into each of your server{} entries in /etc/nginx/sites-avaliable/default or where ever you keep your available sites located.
This setup still allows me to have multiple virtual hosts on the same box behind an nginx reverse proxy.
For this example, I have a DNS A record for examplehost1.com, blog.examplehost1.com and www.examplehost1.com pointing to the same IPv4 address. examplehost2.com and www.examplehost2.com have their own A records pointing to the same IPv4 address.
Nginx will match the incomming server name in the headers of the request. If the server name is examplehost1.com, nginx will send a 301 redirect in the response to www.examplehost1.com. The same applies if the server name in the request is blog.example1.com. When the server name is www.examplehost1.com, nginx passes the request to the specified proxy. The reason I have this is there exists links out in the world with the blog subdomain, but now I want all traffic to end up at www.examplehost1.com.
The examplehost2.com will make sure all of its traffic ends up at www.examplehost2.com.
There are other methods of redirection, but I find this the easiest when you have multiple virtual hosts setup on an nginx reverse proxy.
/etc/nginx/sites-available/default