Woocommerce | I want to the button ‘Add to Cart’ on the (all) product-page changed by just ‘open product’ button

Try this

function wc_shop_open_product() {
    echo '<a class="button demo_button" href="'.get_the_permalink().'">Open product</a>';
}
add_action( 'woocommerce_after_add_to_cart_button', 'wc_shop_open_product' );