Change WooCommerce text strings

functions.php

/**
 * Change WooCommerce text strings.
 *
 * @param	string	$translated
 * @param	string	$text
 * @param	string	$domain
 * @return	string
 */
function projectnamespace_woocommerce_text( $translated, $text, $domain ) {
	if ( $domain === 'woocommerce' ) {
		$translated = str_replace(
			array( 'Apply Coupon', 'Update Basket', 'Coupon code', 'Free!' ),
			array( ''            , 'Update'       , 'Voucher'    , ''      ),
			$translated
		);
	}

	return $translated;
}

add_filter( 'gettext', 'projectnamespace_woocommerce_text', 30, 3 );

Just putting a note, since this scores quite high in Google, that nowadays it would be better to change text strings using translation tools. (even for English)
This would allow for better re-usability and easier upgrades. Good starting point here: https://docs.woocommerce.com/document/woocommerce-localization/#section-11