It’s; just edit the index.hbs file located in: Documents ▸ Publii ▸ sites ▸ YOUR_SITE ▸ input ▸ themes ▸ YOUR_THEME
To preserve your custom code and prevent it being overwritten the next time the theme is updated, please ensure that you have created override files; these are, essentially, copies of the existing theme files that are stored separately from the main theme files → https://getpublii.com/dev/theme-overrides/
Alternatively, you can display the post on the frontpage (look into the theme settings: layout -> front page options) and modify its content to your needs.
