Customise 'add to cart' text on single product pages. Replace 'my button text' with whatever you want it to say.

custom-add-to-cart-text.php

add_filter( 'add_to_cart_text', 'woo_custom_cart_button_text' );                                // < 2.1
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woo_custom_cart_button_text' );    // 2.1 +
 
function woo_custom_cart_button_text() {
 
        return __( 'My Button Text', 'woocommerce' );
 
}