Via Pi-hole's UI, as accessible via Settings | All settings » Webserver and API , or via the corresponding FTLCONF_webserver_port environment variable for a Docker container, or via API calls, or by editing pihole.toml, or via CLI, e.g.
sudo pihole-FTL --config webserver.port "80,443os,[::]:80,[::]:443os"
Just as mentioned in our Introducing Pi-hole v6 announcement. 