Try this PHP snippet:
add_filter( 'generate_navigation_search_output', function() {
printf(
'<form method="get" class="search-form navigation-search" action="%1$s">
<input type="search" class="search-field" value="%2$s" name="s" title="%3$s" />
<input type="hidden" name="post_type" value="post" />
</form>',
esc_url( home_url( '/' ) ),
esc_attr( get_search_query() ),
esc_attr_x( 'Search', 'label', 'generatepress' )
);
} );
Adding PHP: https://docs.generatepress.com/article/adding-php/