Update add-to-cart button text in WooCommerce

Add this code in functions.php

add_filter( 'woocommerce_product_single_add_to_cart_text', 'raza_woo_custom_cart_button_text' );  
add_filter( 'woocommerce_product_add_to_cart_text', 'raza_woo_custom_cart_button_text' );  

function raza_woo_custom_cart_button_text() {

    return __( 'Buy Now', 'woocommerce' );

}

Replace "Buy Now" text with your required text.