duplicate of https://support.advancedcustomfields.com/forums/topic/replace-the_content-to-acf/
Put the code in the OP of that topic inside of an acf/save_post filter https://www.advancedcustomfields.com/resources/acf-save_post/
Get the custom field from ACF and update the post content.
You’ll probably also need to built the actual HTML required for displaying the content on as well like you would in the template file.
