Hi there,
it requires a little PHP Snippet to be added to your site.
This is the filter required for reference:
https://docs.generatepress.com/article/generate_post_date_show_updated_only/
Options for adding PHP to your site is covered here in detail:
https://docs.generatepress.com/article/adding-php/
Quick snapshot of how to for that – there are two options:
Option 1. If you’re using a Child theme then copy the code provided ie:
add_filter( ‘generate_post_date_show_updated_only’, ‘__return_true’ );
And paste into your child themes functions.php file.
Option 2. If you’re not using a child theme then install the Code Snippets plugin:
https://en-ca.wordpress.org/plugins/code-snippets/
Once enabled Dashboard > Snippets > New.
Paste the code in there and save it. Leave all other settings as default.