Same way you do with any other app not specifically designed for it; you start 2 copies of it and put loadbalancer in front of it. I did that via some systemd voodoo

But TECHNICALLY to do that in one without external proxy you'd need to figure out how to set SO_REUSEPORT for the web socket handler, then start the second one before the first.

Haven't actually tried it but someone apparently did: https://iximiuz.com/en/posts/go-net-http-setsockopt-example/

You'd still have any ongoing connections cut unless you unbind socket and then finish any existing connection, which would be pretty hard with default http server.

I just put HAProxy instance on my VPS that does all of that, including only allowing traffic once app says "yes I am ok" in healthcheck. Then the app can have "shutting down" phase, where it reports "I am down" on healthcheck but still finished any active connections to the client.