Change the 'Add to Cart' button text on either single product or archives pages.

<?php

// Change add to cart text on single product page
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woocommerce_add_to_cart_button_text_single' ); 
function woocommerce_add_to_cart_button_text_single() {
    return __( 'Add to Cart Button Text', 'woocommerce' ); 
}

// Change add to cart text on product archives page
add_filter( 'woocommerce_product_add_to_cart_text', 'woocommerce_add_to_cart_button_text_archives' );  
function woocommerce_add_to_cart_button_text_archives() {
    return __( 'Add to Cart Button Text', 'woocommerce' );
}