You can do this at the moment only as a web request if i see it correctly.
Inside the WATO Api is no command to update only the host labels.
If i inspect the button “Update host labels” then i can see a request to the URL
http://<monitoring server>/<site>/check_mk/ajax_service_discovery.py
With request in the following style
{“host_name”:“HOSTNAME”,“folder_path”:"",“discovery_options”:{“action”:“update_host_labels”,“ignore_errors”:true},}