Hi,
I think you have to edit the footer.hbs file (Documents ▸ Publii ▸ sites ▸ YOUR_SITE ▸ input ▸ themes ▸ YOUR_THEME/partials).
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/