add_filter( 'woocommerce_product_get_stock_quantity' ,'custom_get_stock_quantity', 10, 2 );
add_filter( 'woocommerce_product_variation_get_stock_quantity' ,'custom_get_stock_quantity', 10, 2 );
function custom_get_stock_quantity( $value, $product ) {
    $value = 15; // <== Just for testing
    return $value;
}