1. Save a copy of family.yourdomain/robots.txt, download with a browser, it is generated.
2. Edit property "base_url" in file family/data/ config.ini.php to point it to the subfolder instead of subdomain and use https instead of http. Technically this should be sufficient to let it work.
3. FTP the backed up robots.txt file to the root of your domain.
4. Some hosting services have different folders for http (eg. html/public) and https content (eg. html/private) so you might need to move everything.
5. The subdomain record can be removed (hosting control panel) since it is no longer used.
6. You might want to inform search engines like google or bing what the new location is of your sitemap.