There is no way to override it, all you can do is delete the {{jsonLD}} element (helper) from the head.hbs and insert your code manually. To simplify the work you can use JSON-LD Schema Generator e.g. https://hallanalysis.com/json-ld-generator/
To preserve the custom code changed from being overwritten the next time your theme is updated, please make sure you have created override files, which are essentially copies of the existing theme files and are stored separately → https://getpublii.com/dev/theme-overrides/
