[Theme: Child] How to change the function.php?

Hi,

I need to change Child theme image size with this funtion.php code, but I don’t know how can I do that (https://lollum.ticksy.com/ticket/812365).

function lollum_woocommerce_default_thumbs() {
update_option(‘shop_catalog_image_size’, array(‘width’ => 720, ‘height’ => 933, ‘crop’ => true));
update_option(‘shop_single_image_size’, array(‘width’ => 720, ‘height’ => 933, ‘crop’ => true));
update_option(‘shop_thumbnail_image_size’, array(‘width’ => 150, ‘height’ => 194, ‘crop’ => true));
}

Can please someone help me with function.php. Where I need to put this code and is there something else I need to write to get this theme work..?

Thank you!