<?php
add_filter( 'woocommerce_product_add_to_cart_text' , 'custom_woocommerce_product_add_to_cart_text' );
/**
 * custom_woocommerce_template_loop_add_to_cart
*/
function custom_woocommerce_product_add_to_cart_text() {
    global $product;

    $product_type = $product->product_type;

    switch ( $product_type ) {
        case 'external':
            return __( 'Buy product', 'woocommerce' );
        break;
        case 'grouped':
            return __( 'View products', 'woocommerce' );
        break;
        case 'simple':
            return __( 'Add to cart', 'woocommerce' );
        break;
        case 'variable':
            return __( 'Select options', 'woocommerce' );
        break;
        default:
            return __( 'Read more', 'woocommerce' );
    }

}