Solution:
you can find more details here you can add the following to your child-theme functions.php. If you don’t have a child theme you would need to set one up to avoid your customizations being overwritten on updates.
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' );
}