This article describes how to enable http2 in your app, which is using the nginx-buildpack as the app runtime.
Details on the http2 module can be found here:
https://nginx.org/en/docs/http/ngx_http_v2_module.html
To enable http2, you will need to add the flag to the nginx.conf file for your app. An example of this is as follows:
server {
listen {{port}};
http2 on;
}
This should then enable http2 for your app.
Please note that this is specifically for the nginx distribution. For openresty, the list of modules it is built with in the nginx buildpack does not include the --with-http_v2_module