WooCommerce: How to update order quantities at checkout

I am trying to update order quantities at the checkout or when the order has been placed.

I can add custom meta with the following:

add_action( 'woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta' );

function my_custom_checkout_field_update_order_meta( $order_id ) {
   update_post_meta( $order_id, 'My Field', sanitize_text_field( 'testdata' ) );
}

How can I target a specific products quantity meta?

Thanks