Hello there,
You can try adding a filter like this:
add_filter( 'generate_post_date_output', function( $output, $time_string ) {
$time_string = '<time class="entry-date published" datetime="%1$s" itemprop="datePublished">Published on %2$s</time>';
if ( get_the_date() !== get_the_modified_date() ) {
$time_string = '<time class="entry-date updated-date" datetime="%3$s" itemprop="dateModified">Updated: %4$s</time>
<time class="entry-date published" datetime="%1$s" itemprop="datePublished">Published: %2$s</time>';
}
$time_string = sprintf( $time_string,
esc_attr( get_the_date( 'c' ) ),
esc_html( get_the_date() ),
esc_attr( get_the_modified_date( 'c' ) ),
esc_html( get_the_modified_date() )
);
return sprintf( '<span class="posted-on">%s</span> ',
$time_string
);
}, 10, 2 );
Adding PHP: https://docs.generatepress.com/article/adding-php/#code-snippets