Change Host header in nginx reverse proxy

You cannot use proxy_pass in if block, so I suggest to do something like this before setting proxy header:

set $my_host $http_host;
if ($http_host = "beta.example.com") {
  set $my_host "www.example.com";
}

And now you can just use proxy_pass and proxy_set_header without if block:

location / {
  proxy_pass http://rubyapp.com;
  proxy_set_header Host $my_host;
}